Phone Got Locked - 640XL

[Windows Central Question]

Frnds, my phone was working fine till mng, listning music. suddenly music stopped & seen message on screen "Device has been locked for security reason. Connect your device to power source for at least 02 hours and then restart it to try again" pls suggest how to unlock it and what might have went wrong.

 

[Lee Power]

Seen similar before. Device locked & requested that I typed in the characters on screen before it then allowed me to enter phone pin. My phone was on but was in poor signal area, I wonder if a period of time without mobile or WiFi signal causes a Lumia to think it needs to lock itself?