Q: my phone keeps on shutting after a while and wouldnot turn on

after updating to iOS 9.3 my phone keeps on shutting down by itself every now and then and wouldn't turn on ! The bright screen with the apple logo comes , then there is a grey type screen , then it blackens out and the bright screen comes again. This had been repeating for a long time till my phone turns on back. Need help emergency! i tried resetting it a couple of times but nothing seemed to work.