I think you would probably want to have something like a forgot password feature and have it generate a random password and send it to the email address you have on file and of course it would need to update your system so that randomly generated password was now their password until they login and change it.
That is probably quite a bit more coding, but I think that would be about your best option if you're using one way encryption like MD5. Most, if not all of the software that I've used, which uses MD5 for passwords seems to have a feature that works this way.
Another option would be to use
RC4 if you wanted a way to encrypt and decrypt passwords.