all -this happened to me after the 5.1.1 update. My phone FREAKED out and wouldn't allow the fingerprint scan. 40 minutes later I finally hit upon my backup password (I wasn't home - where I had it written down) and unlocked the phone.
First question to you all - does the screen show the "all seeing eye of Sauron?"
Because it appears that with the update, certain settings got turned on in my phone that were NOT previously on - Smart Stay.
if you see the Eye of Sauron, likely smart stay is turned on. Go to your settings, Display and Wallpaper, and see if the check box is checked. Mine was checked on after the update (and I did NOT have it on prior).
Unchecking that has CURED my lock screen issues completely. Fingerprint scanner is working perfectly again.