If Poloniex was hacked or shutdown, what effects would that have on Monero? I suspect they have a ton of coins there, since it seems to be the largest monero exchange. Would it effect the coin in any way besides reactionary price changes?
It would hurt Monero users who rely on exchange wallets instead of offline cold storage.
Volume would quickly move to other exchanges such as Cryptopia, Bittrex, Shapeshift and Bitsquare. Depending on the reason Poloniex shut down, decentralized options such as Bitsquare would likely benefit the most.
No hard fork would even be considered to recover the losses of those who had funds on the exchange. Monero is immutable and such a fork would be an impossibility for both technological and social contract reasons.
From a practical standpoint, there would be no way to trace any Monero stolen from the exchange as the result of a hack. The best chance of finding the attacker would be to trace other coins he/she may have stolen from the exchange and hope the hacker confesses and returns all the Monero after being caught.
A hack of poloniex would most likely not be the end, as it has already happenend back in March 2014 (https://bitcointalk.org/index.php?topic=499580). Poloniex took the repsonsibility and with the help of the increased volume of the new and shiny Monero that hit the exchange, it was able to refund all costumers.