Is the mongrel hung (high cpu?) leaking memory? Check the mongrel logs for the last thing in the log..that is usually indicative of the problem that you have going on. Pay attention to warnings or error messages. Mongrel shouldn't die after simply idling...it is likely your code is causing it to crash in some way.
I'm running ruby on rails with Mongrel clusters serving from the back end via an apache facing the front. The problem that I'm facing is; when I leave the Mongrel without much requests for some substantial time say 8-10 hrs and then try logging into my site, the server does not respond reporting a 502 proxy error (primarily due to a time out) which I found by checking the logs. The site is not for public use as of now, so I cant share the link. I'm unable to track what the actual problem is? If any one can help me why this happens, then it would be of great help. Suggestions to overcome this are also welcome. Thanks a lot.