If you hash your users password (Yay! Well done!) then no, you can't retrieve it if they forget it.
The normal way to handle this is to reset the password to a random string (I use a Guid, but you can use the
GeneratePassword method[
^]) and then email the new password to the users registered email address. Remember to provide them with a facility to change the password, as all random passwords are difficult to remember and thus tend to get written down and stuck to the computer... A simple page which just says "Password reset - the new one has been emailed to your registered email address" is all you need once you have got the userid or email address that they have forgotten the password for.
The email part is pretty simple, there is a generic routine here that should help:
Sending an Email in C# with or without attachments: generic routine.[
^]