Service unavailable problems

I uploaded my web app a couple days ago (first time on a production server exposed to the internet), and I am getting service unavailable messages for both the web app and a plain-vanilla html site.

I've triple-checked to make sure that db connections are closed as soon as they can be (probably a matter of less than a second). All file i/o is wrapped in try/catch blocks. No exceptions being thrown that I'm aware of.

As far as I know, this can occur from db connections being kept open for too long (not an issue) or from too many users accessing the app. I don't know what the max is, but there's been less than ten simultaneous users in the app.

Can anybody offer any suggestions to what may be causing this problem?

Thanks!