I just encountered the same issue yesterday trying to send ethers to TheDAO.
It's an issue with the IPC connection. See my answer to Is the Mist Wallet stable? for details on how to diagnose it (Mist console).
To get Mist working so you can send your transactions, start Mist check the Mist console (Developer -> Toggle Developer Tools -> Wallet UI). You will see a "IPC Connection Error" in red. Mist will retry the IPC connection after a few minutes and will usually start working. You will see a gas estimate on your send transaction screen, or if you browse your watched contracts, your READ FROM CONTRACT and WRITE TO CONTRACT fields will be displayed.
--fast. Was still happening again. Just learned how to send a transaction from the command line instead. – Joe Flynn May 05 '16 at 11:27