You get an error when trying to browse to your SharePoint site: 500.
Then, when trying to turn on logging in IIS, you get this:
Config Error There is a duplicate
'system.web.extensions/scripting/scriptResourceHandler' section defined
In IIS, .net 4 is enabled for the application pool that SharePoint is using.
In IIS 7, double-click on the application pool and choose .net 2 in the drop-down for each application pool the SharePoint site is using. My site didn't come up again until I changed it for all of the Web application pools for SharePoint.