There may be no connection to Microsoft Exchange, either due to connection issue or possibly the Exchange Server is off-line.
You can try to access the site again at a later time when the server issue has been corrected.
Fix the network connection. Assuming the physical or wireless connection is working, have Windows attempt a repair:
For Windows 7:
For Windows Vista:
For Windows XP:
Check out what others say about this issue and review or add your own comments and solutions!