Is there no other member of the Admins group? If not, then you can try (a) finding an older version of the .mdw file
(perhaps a backup somewhere) that still has the Admin user in the Admins group and working forward from there or (b)
opening the database while logged in as the Owner of the database (if it's someone other than Admin) and then granting
permissions as needed. The database Owner can always grant/revoke permissions; that privilege is irrevocable. Of course,
you have to know who the owner is ...
Did you create a new security file? Or did you just use the default System.mdw file as your workgroup? If you used
system.mdw, then you can just find and rename that file; Access will re-create it for you (assuming you're using A2000