I've got a Samsung galaxy j3 and I was in my bedroom listening to my music on my bed which are bunk beds. I sat on the top bunk while my phone was under the mattress and in the metal squares and then my music stopped. So I went to check up on my phone and when I turned it on, my phone went all black except from the top bit and I don't know what to do?
Sounds like the display is either damaged or the connection is loose. If the device is under warranty, contact the manufacturer. If it isn't then you can either try doing the work yourself or take it to a local tech.