I changed the cover on my sons DS last night and got exactly the same thing.
Check all the ribbon connections - especially the 2 smaller ones from the touch screen are fitting snuggly.
Also the ribbon connection where it connects to the pcb near the hinge- check its in square there is a white line on the pcb to help you line it up!!
Good luck - after a bit of checking my sons one works fine now so dont give up hope!!!