Usually the problem is that, with the general search field at the top right, I get too many hits! If I look for "Balloon," I get the pictures with "Balloon" as a keyword, but also pictures that have "balloon" in a sign in the picture and pictures I hadn't realized had a ballon in them. It seems very odd that you're not getting hits at all!
Did you recently upgrade to Sequoia, or to Tahoe? (You indicate Sequoia in your signature.) It takes time for Photos to scan through all the pictures to add its results to the database. Is it possible it just hasn't had time, yet?
If not, then the first test is what léonie suggested-- use a Smart Album to specifically search for "Keyword is balloon" and see if that works. That works differently from the search field (I think.)
Otherwise, to narrow things down, the standard things to try are these:
- Restart the computer (of course)
- Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this:
Start up your Mac in safe mode - Apple Support
- Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens. You can create a new Library by closing Photos and then option-clicking on the Photos icon.
- Make a new user and see if the same problem recurs with it. This is a bit harder, because a new user can't access a different user's stuff. You would need to move your Library out of your own user's folder up to the general "Users" folder.
- Rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon. If you use iCloud Photos, it’s possible that this will cause a re-sync that lasts long enough to make you nervous.
The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening.
Let us know what happens…
