Filemaker databases can become corrupt.
Recommended practice is to make backups of the database on a regular basis, so you can open a known good backup and import the current data.
If there are no known stable database backups you are left with a Recover database option in the File Menu. Using a recovered database sometimes works but is not recommended.
Sometimes, saving a Smaller [save option in Filemaker], opening that, and saving as Clone [save option - no records], and then recovering the clone, opening the recovered clone and importing the data can work.
Sometimes, turning off the indexing on fields, closing the database, opening the database and turning on indexing can fix things.
The FileMaker Pro Consistency Check
This Google "filemaker recovery" gives links to the process
Stellar Phoenix FileMaker Recovery for Mac - $200