Screen popping out on mobile devices is usually due to the battery inflating and pushing the whole display assembly up.
You can simply push the plug at the top right under the screen to reconnect the LCD cable back in if you can. Use home button and power button hold for 10 seconds to hard reset the phone. If you see the apple logo, your LCD is still working. If you don't, you may need to get your LCD replaced and also get the battery checked to see if it's expanded.