This evening, I went to check Mail (it had been opened all day as usual) on my 2011 iMac running 10.10.5 and it is essentially frozen. I see messages with the most recent one from this morning at 6:43am but the messages only show the sender, subject and date - the usual preview/snippet of the message in list view is empty. Clicking on a message does not show anything in the right pane (where the full message usually appears). Clicking on individual mailboxes and the 'on my mac' archive folders does not change the list of messages. Tried force quitting and rebooting but nothing is working. Any thoughts/suggestion?