SOLUTION(?): I had the same issue and there is a workaround/solution that has stopped this from happening for me. So when this happens (and it has happened since the earliest days of the N9, before N9QuickTweaks even, and apparently has many different causes), I usually have to hold down the power button for about 10 seconds to reboot, since you can't see anything once the screen goes black. Once you hard reboot by holding down your power button for around 10 seconds, go to Settings and you'll notice that the light bar is all the way to the left. Shift it a little to the right. Open up N9QuickTweak and apply tweak BB option to Disable Automatic Brightness. Now, I do not use the BB option to control screen blanking; instead I use the FREE app called "Wake" in the OVI/Nokia store. I hope that this helps. This issue is caused because somehow, there is a screen setting that gets set to zero under certain circumstances. It can be any number of circumstances, many having to do with tweaks, displays, phone-ui's, etc., but no one has figured out all possible causes, so far as I know. Since the setting goes to zero when you are on a call and hold the phone close to your face, that is one triggering event. Someone did find the exact file and value that was being reset to zero once, but I can't remember which thread it was. But don't let that stop you from tweaking your device, just realize that when you are using a new tweak the issue may come back up. Just get the phone tweaked like you want it, then re-apply this approach and the problem goes away (at least for me). Good luck.