Symptoms I have had is at first, I thought my power button was beginning to fail. So I started using SLEEP mode to avoid using the power button. Then it began to not wake up from SLEEP.
The screen background went white. My mouse no longer functioned.
CTRL-ALT-DEL did not work. Holding the power button down had no effect.
After one hour, it came out of it's coma and shut down. I went for a second cup of coffee and cam back and his the power button. Nothing. Took another 30 minutes or so so complete the restart. I went to google, slowly it came up.