My nokia lumia 521 screen went completely black after my phone asked to update. How can I fix this

I had this happen on my Lumia 920 a lot. I pressed and held the power button till it finally rebooted. I seemed like it took a minute before it finally rebooted, so don't give up after 10-30 seconds. If that doesn't work try the power button and volume down button till it reboots. Some sites say this is a hard reset that wipes the phone, but as a last resort, I tried it when the power button alone wouldn't "wake it up". When it rebooted everything was normal, so either it isn't a "hard reset" or it doesn't do the "hard reset" when the phone is locked up and the screen is black. I never tried the power and volume down reset when the phone was operational, so I don't know.