It all depends on the version, and what you did. If you ran the security
wizard in a recent version, then the wizard created a backup of your mdb.
Look in the folder where the mdb file is located, and see if there is a file
with the same name, but a bak extension. If you find it, you just need to
rename it to have a mdb extension.
Some versions may have made your secure workgroup file (mdw), the default
one to use for all sessions of Access. The simplest thing to do, is to use
Start, Search and search all files/folders (including system folders) for
*.mdw. Rename every file that it finds e.g.. system.mdw to system1.mdw.
Now open Access and then open your renamed file from paragraph 1 above.
If this doesn't work, then post back with the version, and what you did to
'set up security'.