tldr: Black screen, no display, everything works "under" (I can answer the phone etc). Not a flex cable problem because occurs randomly, regardless of the slider being open or closed. The problem also occurs during boot up, so rather not a software problem.
I think I have something similiar. Everything was fine until a few days ago the screen didn't want to unlock..or at least I thought it was that.
Basically the phone sometimes works OK but sometimes if I turn off (lock) the screen, it won't go back. To be precise, the phone works and if someone calls me I can pick it up moving the finger where the slider should be, it's just the display doesn't turn on. Same goes for booting, bootloader logo etc, so IMO it's not a software problem.
Strange thing is that it just randomly pops back, sometimes when I move the gamepad slider, sometimes when I put in a jack cable or if I hard reset the phone (remove battery). I've had problems with the flex cable in the past and this time I know that it's not it (no difference if I open/close the slider).
My guess would be some power supply or LCD driver IC issues, because the screen stops working almost everytime if I put the phone into the dock. Moreover, if the screen doesn't work, the orange charging LED is not working (green and blue are OK). The moment the screen starts working, the orange charging LED starts working too.
it's very strange but I'm used to electronics doing strange stuff. Just wondering if I should give a try disassembling the phone and trying to fix it or just buy a new one or something?
I might add that I have bought a new battery ~3 months ago, but it's been working fine to this point. I have tried if the same problem occurs with the old/original battery and it's the same.
well, any ideas?
edit: This video shows something very similiar (except the backlight): https://www.youtube.com/watch?v=m4p0-kv1cxA