It would help if you told us what version you are using.

If a recent one, go to Tools, Security, Workgroup Administrator and see what
workgroup file you are joined to by default.  Click on Join and rejoin the
standard system.mdw that ships with Access (search for it first, so you know
its location).

If you didn't create a new workgroup file when you secured it, and have
messed up your system.mdw, try renaming it to system1.mdw.  Reopen Access
and it will create a new system.mdw (if a recent version).

The security wizard may have created a backup of your unsecured mdb.  Look
in the same folder as your mdb for a file with the same name, but bak
extension.  Rename it to have a mdb extension - and you're back to square
one.

If you want to implement security, study the Security FAQ
http://support.microsoft.com/?id=207793

Security Whitepaper
http://support.microsoft.com/?id=148555

Although the whitepaper is old, it contains information to help you
understand security.

I've also outlined the detailed steps at
www.jmwild.com/AccessSecurity.htm