The error listed above in the title is almost certainly some kind of locking issue. The typical cause is that someone who has the database open might have opened up one of the forms, queries, etc. into design mode which will lock everyone out.
Though Access can be used by multiple users there are a fair amount of problems that can occur. Recommending that everyone close the database down sounds easy, but what if you don’t know who has it open? Enter LDBView. This is an old school Microsoft executable that still works against Access 2010 databases (copyright 1995 to give you an idea). This can be downloaded here:
The “open database” dialog for the LDBView lists only *.mdb by default. You will want to change the filter to *.* to see all files and then choose the LDB file that’s in the database with the MDB you want info on.