After putting a new web part in production on our SharePoint 2010 site and running Update-SPSolution to put the new code in place, we got the following errors:
IIS7 Application Pool is unavailable. The IIS 7 Application Pool named . . . is unavailable as the Application Pool has been stopped.
Web Site binding is already in use. The World Wide Web Publishing Service (WWW Service) did not register the URL prefix http://*:80/ for site 1. The necessary network binding may already be in use. The site has been disabled. The data field contains the error number.
The SharePoint site simply displayed a Service Unavailable message.
Rebooting the web and application servers had no effect.
In Sites, the Default Web Site was Stopped, but when I attempted to start it, I got the Web Site binding is already in use error message.
To fix it, I clicked on Application Pools. Two of the Application pools (one for our company intranet site and SharePoint Web Services Root) were stopped. I manually started them, and the intranet came back up.