Is it Filemaker or the database? A new database exhibits the same problem or not?
A database can become corrupted in simple or catastrophic ways.
Saving a smaller version in the Save As options sometimes makes a difference.
Saving a Clone sometimes does the trick.
Using the Recover option under the File Menu will often do it.
Reverting to a previous saved version (saving versions as you develope a database is recommended) often works.
Rebuilding the database from scratch is sometimes the only option.
And if this is a file created by conversion from an older version of FileMaker, check your plug ins. If you have added any plug ins from third party developers, they may not work with FileMaker 12 and may interfere with the correct function of the database. You can drop out all such plug ins and then add them back one by one to see which, if any, are the culprit, then contact the developer to see if they have released an update for FMP 12.