"The Project file '' has been renamed or is no longer in the solution." which was pretty annoying as I didn't know what was happening.
Cause for this error - When the solution can't find the project, it displays a warning when you open the solution and shows the missing project grayed out with "(unavailable)" next to it. The message, in this case, is actually coming from one of the other projects in the solution, most likely a Web project (Website or Web application).
Solution:
To fix it, right-click the Web project and select "Property Pages". When properties page displays, look in the references list for items marked missing.Remove any missing references and re-add them (or) Right click on the solution -> Add -> Add Existing Project - and readd the project which is missing. The solution should then build fine.