My phone goes to sleep (screen goes blank)

Phone still working, rings when a call comes, alarm still goes off but for some reason the keyboard wont wake up - so i cant answer the call or switch the alarm off

Only way to wake the keyboard back up is take out/replace the battery then it's fine again

There's nothing wrong with the keyboard - it works fine whilst the phone is awake - i play games no problem


Anyone eles had this and whats the cure?