If "Service Unavailable" prompt appears, refresh a few times they can access.
This occurs because your site has exceeded the limit caused by IIS.
Because Windows2003 operating system when prompted excessive IIS is not like 2000 prompted "excessive number of links", but prompt "Service Unavailable", this situation is due to the site over the system resource constraints caused mainly occupied resources program too much.
We often use iis7 server monitoring tools to monitor 503 errors.
If a Web site program accounted for too many resources or too many errors occur, the system log will prompt: "Application Pool 'xxx' is being automatically disabled due to a series of mistakes to provide services in the process of emergence of this application pool, or Tip: the application pool 'xxx' exceeded its job limit set time, visit this Web site will prompt:.. Service Unavailable general system will return to normal in about 30 seconds, will be able to refresh a few times a normal visit.
Also, if you visit the site of the current excessive number exceeds the limit iis connection system, suggesting Service Unavailable will appear (too many connections appear under Windows 2000 host will prompt: too many connections, please try again later ; and just direct win2003 host Tip: Service Unavailable).