Re: cant unlock screen
You could try to reset your password using Google Device Manager (log into Google Play using your Google Account and you should see Device Manager on the right).
Unfortunately, secure lockscreens are just that. SECURE. If somehow the fingerprint is no longer recognized and the password input is wrong, the device cannot be unlocked (and more often than not, enough incorrect inputs and the phone will protect its data by deleting itself). Are you sure someone else didn't change your credentials maybe as a joke? I did that to a friend's iPhone once when I figured out his PIN lock...