If you do the same search on the same folder in File Explorer, does it work there?
Also, try doing filename:*.jpg (or similar for an extension that will match some files) as the search. That should be fast and won't involve looking inside any of the files, which should tell us if it's working at all for that folder.
If you type "index" into the Start Menu it should find the Indexing Options dialog that's part of Windows. Click "Advanced" in that and there's a troubleshooting option to delete and rebuild the index.
If you still run into problems, Tools > Find Files in Opus will give you Opus's own search functionality, which doesn't depend on Windows Search or its index and should let you find things.