Short answer

Open System Settings, click Spotlight, and check that file results are turned on. Then click Search Privacy and make sure the folder or disk isn't in the list. If files are still missing, rebuild the index: add the disk to Search Privacy, wait a few seconds, then remove it.

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Spotlight relies on an index, a list it keeps of what is on your Mac. If a folder is left out of that index, or the index falls behind, Spotlight can't show the file, even though it is right there in the Finder. Most fixes below either put the folder back in the index or make Spotlight build it again.

Quick checks first

Press ⌘ Space and type the file's exact name. Apple's own first tip for a missing file is to search its name in Spotlight and double-click the result.
On macOS Tahoe 26 and later, press ⌘ 2 after opening Spotlight. That switches to the Files browse mode, so apps and other results don't crowd out your file. To filter by type, type a slash and the type, like /PDF, then press Return.
Check Recents. Click the Finder icon in the Dock, click Recents in the sidebar, then click the Group button in the toolbar and choose Date Last Opened.
Try the Apple menu. Choose Apple menu ▸ Recent Items, or File ▸ Open Recent in the app you used the file with.
Restart your Mac, then search again.

Not sure which macOS you have? Choose Apple menu ▸ About This Mac. The steps below note where versions differ.

Check Spotlight's settings

Two settings can hide files from Spotlight: the result categories, and the Search Privacy list.

Turn file results back on

Open the System Settings app.
Click Spotlight in the sidebar. You may need to scroll down. On some older versions of macOS it is called Siri & Spotlight.
Look below Results from Apps and Results from System. Turn on any category that is off and that your file would fall under.

Check the Search Privacy list

In the same Spotlight settings, click Search Privacy in the lower right. On some versions the button is called Spotlight Privacy.
Look at the list. Spotlight doesn't search anything in it.
If the folder or disk with your file is listed, select it and click the Remove button (–). Spotlight then includes it again.

Leaving a lot of things in Search Privacy has a side effect. Apple notes that if you exclude certain files and folders, you may not be told about updates for some apps. Exclude your whole internal disk and you won't be told about any updates.

Rebuild the Spotlight index

If the settings look right and files are still missing, the index itself may be stale. Apple's way to rebuild it uses the same Search Privacy list.

Open System Settings and click Spotlight (or Siri & Spotlight).
Click Search Privacy (or Spotlight Privacy).
Click the Add button (+) and choose the disk, such as Macintosh HD, or the folder you want to reindex.
Wait a few seconds.
Select the disk or folder you just added and click the Remove button (–).
Click Done and quit System Settings. Spotlight reindexes that disk or folder.

Reindexing a whole disk can take a while, depending on how much is on it. If you see an "unknown" error or a permissions error when you add the disk, your user account may not own some items on it. Apple's article links to steps for changing permissions.

Advanced: check indexing in Terminal

Only if you are comfortable with Terminal. macOS includes a command called mdutil that manages Spotlight's index. To see whether indexing is on for your startup disk, run:

mdutil -s /

Its manual says mdutil -E erases the index for a volume so it can be rebuilt. Run it as sudo mdutil -E / for the startup disk, and enter your password when asked. This does the same job as the Search Privacy steps above, so try those first. Type man mdutil in Terminal to read the full manual.

Files in iCloud and on external drives

iCloud Drive and Optimize Mac Storage

With Optimize Mac Storage on, macOS can keep some files in iCloud when space runs low. Recent files always stay on your Mac. Older ones may only be in iCloud until you need them.

Open a Finder window and click iCloud Drive in the sidebar.
Look for a status icon next to the file. In iCloud means it is only in iCloud, and you need an internet connection to open it. Downloaded files show no icon.
To see status for every item, choose View ▸ as List, then View ▸ Show View Options, and select iCloud Status.
To get the file back on your Mac, Control-click it and choose Download Now.

Turned on Desktop and Documents in iCloud Drive at some point? Then files from those folders live in iCloud Drive too, so check there.

External drives

Connect the drive. Spotlight can only show files from a disk that is plugged in and mounted.
Check that the drive isn't in the Search Privacy list. If it is, remove it.
To reindex just that drive, add it to Search Privacy, wait a few seconds, then remove it, as in the rebuild steps above.
A Time Machine backup disk is a special case. Spotlight keeps indexing it even if you exclude it, because Time Machine needs that, but searches skip the backups stored on it.

Search with Finder criteria instead

Finder search lets you filter more precisely and shows results in a window you can sort. It also helps when you don't remember the name.

Click the Finder icon in the Dock to open a window.
Press ⌘ F to start a search.
Click the Add button (+) on the right, below the search field.
In the far-left pop-up menu, choose a rule such as Kind or Last opened date. Choose Name to find items whose name contains a word. Pick Other for more attributes.
Add more rules until the list is short enough to scan.

You can also type kind: in the search field. For example, invoice kind:pdf.

Check where Finder is searching, too. Choose Finder ▸ Settings, click Advanced, and look at When performing a search. It sets the default: your entire Mac, the folder that's open, or the scope you used last time. If it's set to the current folder, a search started in the wrong folder will come up empty.

When it's still missing

Open Downloads with ⌥ ⌘ L in the Finder and sort by date. Many "lost" files are there.
Click the Trash in the Dock and look there.
Search for a related file you can find. Then look at what sits next to it in the same folder.
Don't remember the name at all? See how to find a file when you forgot the name.

Search the folders you care about in plain words with FileShelf

FileShelf is a Mac app that keeps its own index of folders you choose, such as Downloads, Desktop and Documents. Because that index is separate from Spotlight's, a stale Spotlight index doesn't affect it. You type the way you'd say it, like “pdfs from last week over 5 mb”, and results show as big previews. It can also read the text in screenshots and images when you ask, so you can find one by what it says. Everything stays on your Mac.

It is not a Spotlight replacement. It only searches the folders you add, and it doesn't search apps, settings, email or the rest of your Mac. If Spotlight is broken system-wide, the fixes above are still the way to repair it.

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