Sometimes, I will be using the Nokia Lumia 521 and all of a sudden, the screen will go black and then the phone will just...

Hello Marko, to resolve your problem, you should first try turning your device off then turning it back on again. If this does not work, try removing the battery then replacing it. If the device is still unresponsive, you should perform a hard reset by holding the volume down button and power button simultaneously until the screen goes black. For more detailed instructions, check out this Trouble Shooting Page. Good Luck!