Broken Sleep Mode?

Normally, I press the sleep button on my computer (laptop), and then the monitor (screen) goes black (turns off) as the system (computer's operating system) goes into stand-by mode, AKA it goes to sleep.

Awaken

Normally, my computer wakes up when I press the sleep button again. But every few days, it doesn't wake up, which forces me to press and hold on the power button for ten to twenty seconds straight to make and force the computer to turn off completely.

Restart

After that, I restart my computer, I mean, I turn my computer back on. I'm feeling this might be a bug in Ubuntu Mate, my operating system, but I am currently not sure what it may be yet.