Not sure when this message is popping up for you.
Still, have you tried logging out and logging back in? Also, when not in your DB, see if you can track down the ldb file on your PC -- this will have the same name as your database except the extension will be different. If your file name is MyFile.MDB, your security file's name will be MyFile.LDB. If you can find this file when you're DB is NOT open, delete it. And then see if you can get back in.
Also, if this fails, open a new Access file and import all from the old file...
Regards,
Tim