I have LTS Ubuntu 12.04 and when I close my lid the screen goes black but doesn't suspend because I can feel it getting warmer as if it was processing something. When I open it back up after an hour, I can see that all my battery is drained.
sudo apt-get install laptop-mode-tools
I had the same problem listed here. When the laptop lid closed, the screen turned black and the fan was still on and didn't resume when I opened the lid. After I installed this package, it suspended correctly and resumed when I opened it.
Go to System Settings > Power . From there you can change this. I'm attaching a photo about it.