There's a chance you may have damaged the battery connector. Try plugging the
iPhone into a computer, if it's recognised after a few minutes you could have been sold a dodgy screen. Or it could just be that the process has completely drained the battery and could need charging for a bit, try plugging it in for around 10 minutes, if you don't get any response unplug it to prevent any further damage. Or take the battery out and check the ribbon, there's every possibility that you could have torn it, even the slightest tear could render the battery useless. As well as this, make sure that all of the connections are properly seated at their points.
Hope this helps.