Exchange 2010 mailstore dismounted, and there is not possible to mount it again, and i have to restart server. When logging in after restart server hangs with "applying internet explorer branding policy" after 2 or 3 restarts server comes up again.

Initially I would look at the applicaion and system event logs. Can you let us know what you see logged in there?

If you can't see so much information, start the diagnostic logging for MSExchangeIS Then check the event log again and post it in the thread Jonas Andersson | Microsoft Community Contributor Award 2011 | MCITP: EMA 2007/2010 | Blog: http://www.testlabs.se/blog | Follow me on twitter: jonand82