Hi,
Without knowing exactly what you did when you were first "playing around a little bit" it's hard to say.
First off, you can find which workgroup you're joined to by running the Workgroup Administrator (C:\Program Files\Microsoft Office\Office\1033\WRKGADM.EXE in Win 2K)- as a default, you want to be joined to C:\Program Files\Common Files\System\SYSTEM.MDW
BUT there are two golden rules when working with Access security - take backups, and read and understand the principles. There is a lot of reading, but it's worth the effort - in fact, it's the only way. If you don't understand what's going on, one day it will all go horribly wrong. Sorry, but you just can't do Access security 'quick and dirty'....
Once you have read up on the links in my last post, you should be able to work out what you've done, and therefore how to recover from it.
Hope this helps,
Bob.