Are there numerous people using the database simultaneously? did you have a crash, or irregular shutdown?
When you go back into Access, under the Tools menu, go to Database Utilities and select "Compact & Repair Database". This may help. If you have VBA code, try re-compiling that as well.
I have to do the above things occassionally, and I am not sure why. It may be because people who are using the database are shutting down their computers incorrectly. I'm not sure why it happens. but those fixes usually work for me.