belodion is correct--when you set up your fingerprint lock, it required you to select an alternate PIN, which is different from any other PIN or password you may have selected for other forms of screen unlock. So you need to remember what you selected as an alternate PIN. This kind of information is very important to remember for just these kinds of situations, because the fingerprint unlock seems to fail with some regularity, based on the posts in these forums.
If you just can't remember it, then you may be forced to do a factory reset: http://support.t-mobile.com/docs/DOC-10404
. That will unfortunately mean losing your locally saved data, so hopefully you had been backing up or syncing data on a regular basis. In the future, if you're prone to forgetting important passwords, consider writing them down and keeping them somewhere safe at home. Good luck!