If you want an IMHO, you'd be better off stripping down and removing the battery. Then try turning it on without the battery in (To clear any voltage left in the board), followed by leaving off for 24 hours or so to let the memory clear.
Try booting again after that and see if anything happens. I'd also try the factory reset sequences with and without the SDCard in, and see if that gets you anywhere. (If you get a light sequence, post it here even if the screen does nothing)
It's always possible the screen (or software) has decided to die at this time, and was completely unrelated to your U-Boot changes, and TBQH as it booted originally, I'd suspect this to be the case. A broken U-Boot should normally result in a brick no matter what, not something that dies a couple of days later