In the past few months, I have had random situations where a server would restart, and following the restart, no one could connect in or the server would not connect out. Normally, a second reboot would fix the problem.
I have discovered that this is caused by the application of the MS08-037 patch. If the randomly assigned ports conflicted with the IPSEC service, this service would fail, causing the server to start in block mode where all network connectivity to the server is blocked. The proactive solution as documented by Microsoft in http://blogs.technet.com/sbs/archive/2008/07/17/some-services-may-fail-to-start-or-may-not-work-properly-after-installing-ms08-037-951746-and-951748.aspx is to modify a registry entry as follows.
Add the following port ranges and reboot.
There is more information on this problem along with other scenarios which I have not yet encountered.