Make sure that Battery is fully charged.. Keep it connected to the charger for about 10 minutes and see for difference..
If problem remains,..Remove the Battery for couple of minutes.. Insert and check..
Else, try the Hard Reset ..Press and hold *, 3 and Green Button and switch the phone ON.. Leave the buttons only when you see the Nokia logo.. This reset will however remove all data from the Phone memory. Hence, in absense of a back-up, you tend to lose it all..
If the phone is getting detected by the PC..see if you can take back-up before the reset.