Here are a couple of possibilities:
1. Check to see if there is a .ldb file (Access icon with padlock in upper
right-hand corner) in the same folder as the location of the database. This
is the associated record-locking file, and if Access crashed for someone with
the DB open, it could have been left in place even though it is not open. If
so, just delete the .ldb file. Just don't delete the database accidentally
(.mdb).
2. If not #1, is the .mdb file and/or the folder in which it resides
read-only?