I get visual snow when I get a migraine. These migraines are called "silent" migraines, because I don't get a lot of pain, only a mild headache and vague "hungover" feeling for a couple of days. I've been getting them since at least the seventh grade, and they're harmless (but definitely scary). The visual snow is apparently caused by some (mild, harmless) swelling of the blood vessels near the optic nerve. Sometimes the snow only takes up a little of my field of vision, but usually it starts in one eye and spirals over into the other. It's usually completely gone in about 20 minutes. Whenever the "snow" starts, I take a Tylenol, and put something cold on the back of my neck. Every time I get one I worry that it won't go away, but it always does! Try to relax and let it run its course, and know that you're ok.