The network has been exploited by a hostile party who is currently manipulating it. They are preventing proof of stake blocks as well as disrupting the difficulty calculations.  Proof of stake blocks were working as of 5am 4/22/15, after the patch was released. 

The POS blocks stopped and the transaction structure of clients was rejected by the network as being invalid.

This entity is responsible for the massive hashpower and demonstrated it 24 hours ago when their hold on the network was threatened. 

They are also now in possession of over 900k coins they got by destroying a network launched on behalf of a charity.

We have asked Bittrex to freeze the market as we develop a solution.

There are many options available to us to remedy this situation, and we are sorting through them all.