It’s not a nice place to be I’m sure you’ll agree!
Before we go on, I can’t guarantee one size will fix all!! This worked (well, fixed) our setup – it’s by no means the only way to solve the scary place of no working site.
In our setup we make use of the IIS URL Rewrite module. Under the hood this maps some xml within the web.config to the rules it applies. If for any reason you’ve goofed up this config, easily done with rogue config transforms or general typo’s, then you may have broken this IIS feature. The giveaway, when you double click the feature in IIS you receive an error message.
The reason for posting – this took me a while to track down the first time, now it’s my go-to verification if we don’t get any Sitecore logs or Event log entries.
The issue here isn’t really related to Sitecore, the lack of Sitecore logs is simply another symptom of the problem.