This happened to me, xD i had a broken LCD Screen and i repaired myself and then for extra protection i bought a case and a Screen protector thing and it started acting Cazy!! i got scared but then i took it off and tested it again and the Screen is working great!
Yeah, that scared me also. I traded a 360 rgh slim for a launch Wii U basic. The guy had a screen protector on the gamepad. One time, it did nothing at all. I could see the screen but it responded to NOTHING. In anger, I "touched" it a little hard. Not enough to hurt anything. Tapped it with fingertip. Then it hit me, "Take off screen protector, it is not flush on there with how the guy put it there." BOOM, worked again. I would imagine dropping it would show damage to the actual screen. You would see it.
One time my daughter pressed a plastic "pen" onto her "Learning pad" somebody bought her. The plastic over the screen shattered. She did not drop it, jump on it, or anything else. A plastic pen, 4 year old pushing on the screen while sitting down. I wrote to the company and they replied angrily stating, "She dropped it and it is not our problem." I then wrote back, "I never said she dropped it...here is what happened...." I received another reply apologizing and stating as a one time courtesy they would send another one. They had me ship the old one (mailed postage and everything for me) and sent a newer model back. I told them I would tell everyone I know and rate them as a horrible company. A little word can spread a long way. They rectified it and I say vtech really came through. Now this crap? You modded it, and you chose to not do your research and sent in the entire unit. Learn from it, but there are no other options.
Maybe a nice rep would side with you if you say, "I had a gamepad issue, sent it in for repairs, and my unit came back as a brick, it won't work at all." That MAY get you through if you are nice about it. If they have a record and documented it was a modified system then you are SOL.