A user cannot logon to a Windows 2008 R2 Server, other users may be connected and be able to reconnect.
This issue is recorded in a Microsoft KB Article http://support.microsoft.com/kb/2661332 and it is caused by an issue with Csrss.exe and Microsoft Excel or Visio.
Microsoft KB Article
There is a hotfix for this issue, request and install the hotfix. I did this while the server was in production, not best practice but I had little choice as my customer would not tolerate a reboot at that time.
To enable the hotfix I restarted the Remote Desktop Services Service, often this is all the reboot does when it is requested.