Double tap suddenly stopped working on my N9. Also it doesn't show time anymore on stand by screen. I noticed this while charging my phone. Saw that the screen of my phone has went black. Picked it up, tapped the display for numerous times, nothing. Then woke it up with on/off button.

This is really disturbing, because i'm so used to the double tap function and stand by screen.

Any ideas?

Sounds like a broken proximity sensor. Maybe someone with experience with that specific piece of hardware can tell how to fix it?

And it doesn't show who is calling also, screen just stays black.

proximity sensor, exactly.
first try to clean top of screen glass, check for scratches, etc.
second:after every reboot

definitely prox sensor cable loosen,u drop it hard?I experience that n manage to fix it by opening the fone (t5 torx) n gently reattach prox film cable(double sided tape too)..check out le55ons from ytube..helps u visualize the whole process..hth

Also,
Have you installed a tool such as GConfik? I've noticed that it somewhat breaks the proximity sensor functionality, as it does not detect when the object on top of it is removed.

@Shadwblade2652 you wrong here.

Installing GConfik prevents the lock screen from showing after the proximity sensor is blocked. It happens to me, and I believe it happened to another user as well with the same issue.

"Notepad can break and destroy your system. Don't install Notepad and remove it as soon as you read this message. Notepad can open text files, and you know most of config files are texts" - your sentence just like that for me

And just for note: gconf doesnt keep any settings for proximity sensor. You mixing it with ambience light sensor