In the back pocket you may have damaged the lcd. I would try a Home+Power to reset the phone. If that doesn't work, you can try powering down, lifting the glass and re-seating the cable connections. Also, from what you stated, it is plausible that when you sat down there could have been a coke can, or anything behind you that could have spilled into the phone and dried out without you even noticing (but, in that case I would suspect many more issues popping up).
If none of the above works, order a new screen and give it a try. But, honestly, when I typically see a white screen AND banding, and reseating the cables doesn't work, 99% of the time it's a faulty lcd (or in the case of replacing the screen a cable accidentally gets crimped/damaged). :(