Why does my MBP sleep on Mountain Lion and often not wake up
I had a similar problem with my late 2012 MacBook Pro. It wouldn't wake after a long sleep: black screen but system running (I could access it via SSH).

Before trying any solution, I recommend you to reset the SMC and check if the problem persists.
Also run pmset -g in Terminal (it won't change or break anything) and take note of your current settings.

I solved this problem disabling hibernation, which was introduced by Apple with a firmware upgrade. I did this by running this command in Terminal:

sudo pmset -a autopoweroff 0


To reset the original setting:

sudo pmset -a autopoweroff 1


The autopoweroff option is undocumented by Apple (doesn't appear in man pmset); it seems to be related with the "Deep Sleep" mode.