If I understand you correctly your phone is not really turned off but the screen stays black. So when you call your phone it still rings?
If that happens since you updated to 1.6.2 please continue here:
Alternatively your screen might not be connected well. Try to take it off and clean the connectors carefully. When reassembling make sure the connectors align well and everything sits tight. Also if the issue occurs and carefully bending the phone or putting slight pressure on the back cover where the connectors are indicates that it’s a connection issue.