Failed to start the BizTalk Host instance. Check the event log on the server for more details.

Have you done any changes to BizTalk config file recently. Can you open the file in any XML editor any check it has any format error. Also check for a custom entires, which may not be vaild. I would have taken backup of the config files before any chances, if you have then compare the changes. If you have done some changes make sure you have done them in the 64bit config files.