What version of Access? Did you run the security wizard?
If you deleted the bak file (are you sure you did this?), check the recycle bin and see if it's there. Check your backups - maybe there is a copy of the bak file there. Or go to a backup that has your mdb before you secured it.
Failing all that, do you have a desktop shortcut to launch your secure mdb? Can you use that and successfully log in? Try usernames you set up using the security wizard. Also try the usernames using a blank password.
If you don't have a desktop shortcut, use Start, Search and locate all *.mdw files on your computer (include system and hidden folders), and note the paths.
Open Access, and go to Tools, Security, Workgroup Administrator and see which one you are joined to.
Post back with your progress.
