Mac Mini crashes with 'snow' screen

It's already happened three times, this time (today) I had a hard time to restart it. Basically I am working and suddenly all I see is the screen full of billions of B&W animated dots like random noise similar to the old TVs when there is no signal.

It doesn't even power off, I have to unplug it. I power it back on it restarts okay (but today it took several attempts, it would keep restarting with the same screen noise, no attempt to even start booting).

it's a Mac mini late 2012 running 10.10.3, Intel Core i& 2.3GHZ, 16Gb RAM.