Hi,

First try with a Factory reset, but this will erase all your data:

Samsung Galaxy S5 - Password, Screen Lock Removal

If this not help you, the next step is to reflash the firmware. Go http://www.sammobile.com/ and make a registration, then search fro the corresponding ROM for your device. In the download page you will find instructions and a link for Odin app (you need this app for the reflashing procedure).

If this not help you, then you have a hardware problem. The phone have to be opened for an inspection.

Regards.