When your Mac doesn’t wake from sleep, the reason could relate to uncooperative hardware or unstable software. When people don’t wake from sleep, it could be the sign of a Monday morning or an upcoming exam…
Sometimes when my mac goes to sleep it doesn’t want to wake up and the screen stays black. I have to turn off computer then turn it back on. Why does it do that?
Sometimes a USB or FireWire peripheral can prevent a Mac from going to sleep or waking back up. See if things work correctly with no external drives or printers attached. Also run Software Update to get all the latest patches for your Mac.
A crashed or hung program can also prevent a Mac from waking up from sleep. Check your system logs via the Console application in your Utilities folder. Look for any entries about what was running between the time the Mac was last running correctly, and when you found it frozen. You may have a program or background task which needs to be upgraded.
If none of that identifies the culprit, you may have something corrupt in your OS X installation. In that case make a backup of your data, then try reinstalling Mac OS X.
Readers, have any additional suggestions on this topic, or corrections/clarifications on the advice above? If so, please leave your thoughts in the comments.