Hi, my ipad 2 just got the same problem and after reading all relative post, finnally i figrue it out, that is no software issue at alll just hardware,
I did attemp replace the power button cable on my ipad so i must disconnect the back camera cable so there is my problem. Because the connect some how not exact in place, i took the camera out and the issue is gone. So, in your case, if you did try to replace anything look at them again or you just take them out one by one and reboot ipad until it work.
Hope this help