In that case, try removing and replacing the battery. If no good, and if you can boot into Recovery Mode, try a Cache Partition wipe. It may fix the problem and will not cause a loss of data.
I am going through this problem right now. Contacted Google and with the upgrade to Lollipop Android 5.1, you can't keep putting in the wrong pin/password. It won't prompt you to change it. Have to reset/recover and not touch the phone for 72 hours.