I believe it is the LCD. If it is the lcd then press and hold the power button and go make a cup of tea. Then come back and try talking to siri. If siri responds then give the cable area a good wack. If this does not solve it then try reconnecting the cables (Take it off and then put it back on). If it still doesn't work then check the screw that fell out and compare it to iFixits guides and put it where it belongs. Final resort: Go to the local repair shop and ask them to check it out. (Make sure the shop is not apple or you'll get sent off with a smack in the face. kcuf apple for not making things user-replaceable.).
Hope I helped.