Symptom: coming back from sleep mode the screen is black and no mouse cursor can be seen. The light in the Apple on the lid of the laptop will light, but other than that, the system is unresponsive.
Solution: try clearing the NVRAM (Non-Volatile RAM)
Hold down all of these keys: Command, Option, P, and R, and the power button. Hold the keys down until you see the screen flash. Repeat the process a second time. Then release and power up normally.