This is the dreaded "intermittent bug" unfortunately.  Though it DOES happen to me rather often (usually at least once a day, and almost guaranteed to happen if Evolution has been up and running for more than 8 hours or so).

Basically, Evolution tries to do something IMAPish.  In my case, I've noticed this when it scans for new mail and tries filtering new messages or when it tries to access a remote IMAP contact list.

In the former case, the progress bar displays something like:

0% Filtering New Messages, and Evolution goes off into the woods and never returns.  I've let it sit in this state for several hours before and it never recovered.

The latter case is when it is trying to auto-complete an e-mail address while creating an e-mail message if it has to access a remote IMAP contact list.  IN this case, I'll start typing out the name, Evolution will try to access the IMAP directory, and never return.  

Both cases result in Evolution hanging with no recovery, requiring me to kill it.