Why Has Your Application Pool Been Shut Down By IIS
When an application pool exceeds a certain number of errors in a given time, the Windows web server engine (IIS) will shut down the application pool (rapid-fire failure) to protect the performance of the rest of the server.
If multiple sites share the same application pool - a common configuration with virtual hosting, as running each site in its own application pool uses more server resources- then it could be any site within that same application pool which has caused the problem. If only one site runs in the application pool, then only this site can cause the error count to exceed the quota.
We can tell if IIS has shut down an Application Pool due to code errors by logging into IIS and looking at the Application Pool which is not working. It will show 'Stopped' as the status (other Application Pools on the server will still be working ok and will show as 'Started'). As an additional test we also test the default site.
If when we checked your site:
A: Your application pool had been stopped by IIS and others were still running.
B: Only your site ran in this application pool.
Then the problem can only have been caused by code running in this site (due to the way IIS works), so this matter needs to be passed to the web developers of your site for investigation.
We would only recommend investigating the problem if this happens more than once in the same month, as it may be that different pages returned errors at the same time and these do not arise at the same time- it just happened on this occasion that the number of code errors exceeded the normal threshold.
Error thresholds can be increased slightly if it happens occasionally but this should not be done unless you get this error no more than twice per day and no less than twice in the same month.