A weird thing is happening at the moment between the Ethereum network and some major exchange.
A friend of mine withdraw some Ethereum from Poloniex to his Mist wallet yesterday and the transaction "disappeared". Poloniex did not give him a Tx. He wrote to support and got no answer yet.
Today, Poloniex and Coinbase wrote that the Ethereum transactions will be delayed. (Coinbase : https://status.coinbase.com/incidents/wvhcfrftmdlx and for Poloniex it is written when you want to deposit/withdraw the coin on your account).
Is it a coincidence that this happens in the same time for Coinbase and Poloniex ?
I found another user in difficulty and no answers yet : https://ethereum.stackexchange.com/questions/18460/missing-ethereum-transaction-from-poloniex-to-myetherwaller.
I would be glad to discuss in comment with you what could be the reason for such an event and if you know if any other exchange is hit.