Have you, by any chance deleted the template your home page uses? Or has anyone edited its template recently?
With version 3.1 SP2 you may get this error at the Pages > Site Map tab, when you select a page whose template has been deleted. The only way out of this is to delete the page. In version 3.2, the error that is displayed in this case is more informative, it states:
The file '/[YourProjectName]/App_Master/NameOfMsterpage.master' does not exist.
The error can also be displayed is when the <form> tag is missing in the master page.
The error is thrown at the Pages > Templates tab, after clicking Edit for a template.
To eliminate the problem:
1. Open the .master file your template is using.
2. Make sure that it contains the @ Master directive, as well as all of the top-level HTML elements for a page, such as <html>, <head> and <form>.
For more details, please refer to the MSDN library: ASP.NET Master Pages.
If your case is different, please give us more information about the steps you took before the error occured.
On a side note, did the Value cannot be null.Parameter name: virtualPath
error go away and did you upgrade to the latest release>?
Looking forward to your reply,
the Telerik team