I installed CM7 on my LG P990 five months ago and it worked well. Yesterday, I was watching a video on YouTube and my phone turned off suddenly. It didn't want to turn on, and then my battery dropped to 0%, even though it had been at 100% a minute earlier. I connected my P990 to a charger, and then my battery percentage indicator was at 100% again. Why? I hate when my phone turns off and black screen of death until I plug a charger in! I did a data wipe/hard reset the but problem is still there.
There are 2 possible problems and solutions:
Check and replace your battery. Leaving your phone on charging overnight can sometimes cause this problem.
Install CM7 again. This is the most probable problem/solution. Rooted apps or CM itself sometimes lead to this problem.
XDA also might have some of the solutions that you want.
Best of Luck.
PS: If you think that replacing your battery or installing CM again is too much of an effort, You might want to chuck your phone and buy a new one :-)