Nobody panic. Steem is hardforking in 35 minutes. Poloniex disabled deposits in preparation for the hardfork. In all likelihood, they have already upgraded their witness_node, and are currently testing the new version to make sure no bugs in relation to their backend were introduced. Deposits and withdraws of XMR also get disabled in anticipation of a scheduled hardfork, and are reenabled after - since a hardfork carries a risk of a chainsplit (however small), to avoid the (however unlikely) possibility of double spends exchanges will disabled deposits/withdrawals of a coin when it is forking. Steem and SBD are also disabled with OpenLedger right now. Just sit tight, everything should be back to normal in a few hours.