The Earth has likely been fully enveloped in ice and snow before, you should know that there were several ice ages and the one our ancestors survived was one of the smaller. Damn if we had another snowball earth we would have never existed. When a megavolcano erupts it can spew it ashes so high that it envelops the whole planet and blocks sunlight and that is what causes cooling on a global scale.
A Snowball Earth, or at least strong glaciation similar to during the Ice Ages, is thought to have happened over 600 MA ago, but the circumstances for that were very special, as the landmasses all came together and formed the supercontinent Pannotia right above the south pole. In other words, the landmasses froze over - but the seas didn't fully freeze. What's interesting is what ended it even before Pannotia fully broke up: Once frozen Methane released when it started getting warmer, which caused additional warming and additional Methane getting released in a continuing circle, something that's also starting to happen right now.
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454
The Steam/GOG key gifting thread: https://gamrconnect.vgchartz.com/thread/242024/the-steamgog-key-gifting-thread/1/