The first thing you should do is make a copy of the database. This will keep you from having to start completely over if something else goes wrong.
Next, try running a Compact and Repair on the Database (if it's a Split DB, be sure to wait until nobody's using it before you try and Compact the backend).
If that doesn't solve the problem, then I'd try re-compiling the database. From a command prompt, go to the Folder where you have Access installed and run the following command:
Code:
MSAccess.exe MyDB.mdb /decompile
Make sure you enter the full path to your database or you'll just get an error.
If neither of the above methods work, then there's most likely a section of the VBA Code that broke somehow. You'll need to open the Form in Design View and step through the Code until you find the error.