I have occasionally the same problem as idro. Screen just won't turn on when double tapped or when power button is pressed. I have my own workaround. The screen will turn on if there's a notification of any kind.

I just tap the statusbar and press (from memory) to activate psm and voila!

I have no idea when the problem started or how it is activated. Sometimes it simply happens after normal usage but not always. I have no idea.