plz answer this...

i put my iphone 4 on charging on UPS it was functioning well but i saw it after few hours its screen turned black, it is getting calls n txt messages but not showing display not sensing touch... but when i take a sanp with screenshot it turned on its screen at that time n then switched again black... when i put my iphn on charging it got charged but not showing display...plzz tell me wot to do....