If your Access database starts prompting you for a password even though you
never set one, it usually means that the mdb file was opened in another
application, most typically Microsoft Word.
The mdb file structure, specified by Jet, is very specific, and a
re-arrangement of even a few bytes could render the database corrupt; and a
Save operation can rearrange the bytes.
Microsoft Word sometimes does not require you to specifically
hit the Save button; because of background save option, macros, and
probably some other settings, Word can automatically force a save without
informing the user. And if the file is a Jet database, this save
operation could rearrange the information in a manner that forces Jet to think
that a database password has been set.
Unfortunately, there's no workaround, other than to never open the
database in anything other than the correct version of Access. If your
database is corrupt in this fashion, your best bet is to contact a data
recovery service, such as PK Solutions,
or restore from a clean backup, depending on how critical the data is.
For more information, view this Knowledge Base article.