Websitepanel reads a site’s web.config file to pick up on any custom settings.
A problem can occur if there is an error in the web.config uploaded to the site, or, if it’s for a different version of ASP.NET that was configured in Web Site Settings.
The issue is that once you have a non-working web.config file, you cannot get into your Webpanel settings to make any changes.
Should this occur, the easiest fix is to temporarily rename your web.config file to something else so it won’t load, then go to your Websitepanel settings in your browser and make any appropriate changes (such as changing the version of asp.net to the appropriate one for your code)