I was looking at old photos on my external hard drive. And when I clicked on one and opened it worked just fine. However the second after i clicked the close button, my screen went grey and started to basically act like a restart out of nowhere? The apple logo never came on or anything. But it started the spinning wheel that pops up when it turns on. Then it proceeded to just basically act like it was restarted all in the span of 15 seconds or so. Re-opening apps I had open, opening the ones I like to have open upon a start up, etc etc. Anyone have this issue before? Is it serious? What was the cause of it?