In addition to what Patio just said, I was typing this while he posted.
Mibbit's password management is handled outside of Computer Hope so I can't really do to much other than recommend the general suggestions when passwords don't work.
- Make sure your caps lock is not on, passwords are often capitalized.
- If password contains numbers make sure num lock is on.
- Make sure you're using the right username
Finally you can get the IRC server to send you your password (assuming you used a valid e-mail address) by typing the below command in the IRC server tab.
/msg nickserv sendpass nickname
Hope that helps