I recently upgraded my Macbook Pro late 2010 to Mountain Lion (from Snow Leopard). Now it has this problem where it will sometimes sleep and not wake up. However, I hear that the computer is still on, since the fan is spinning, and so is the hard drive.
This happens intermittently, and I can't seem to isolate conditions of when it sleeps and wakes up, versus sleeps and doesn't. The battery is fine in all scenarios. Sometimes this happens when the MBP is plugged in.
The one constant thing I notice is when I close the lid, the front light will remain lit (but when it sleeps, the light will flash slowly). Then when I lift the lid, this light will go out.
The only way to get it to snap out of this "fake sleep" is to hold the power button until it shuts off, then start the computer again.
What's going on, and how do I fix it?