Today I experienced an error on a VM “server unavailable” when I tried to navigate to any page on a web application. It turned out that the app pool under which the web application ran had crashed. To correct this I simply restarted the app pool in iismanager. Note that an iisreset or reboot of the vm did not restart the app pool.
I found this related article for wss2, but this seems to apply to all versions of sharepoint: http://support.microsoft.com/kb/823552