Firstly, not to worry. Apparently someone unloaded a ton of Ethereum coins on the GDAX(Coinbase) exchange today for $13 a coin! Perhaps this was a mistake, or a hack, but I doubt it was on purpose. The exchange immediately resumed normal exchanges around the $310 area.
https://www.reddit.com/r/ethereum/comments/6io8u1/what_the_fuck_just_happened/
Ethereum also received a bit of a difficulty reduction today, as a few miners probably got concerned and switched to mining other coins.
https://www.coinwarz.com/difficulty-charts/ethereum-difficulty-chart
I'm excited for the low prices of ETH, and looking forward to it going higher as the difficulty increases.
Full disclosure: I'm an ETH miner, and a long term HODLER. Blah Blah do not take my opinion as trading advice.