Try using the Utilities > Repair A Poker Database option to fix the file.
If it works for a while after the repair and then comes back then this error means you have hit the maximum allowable file size for an Access database file (imposed by Microsoft, not PT) so you need to start a new database using the File/Maintain Database Names window and then start importing hands into that.
Another option is to convert the database to the PostgreSQL database format that does not have the file size restriction. See this post for more details.
http://www.pokertracker3.com/forums/archives/PT2/viewtopic.php?t=6310