Moved from Home to work and went to fire up my project and after long pause
"The provider did not return a ProviderManifestToken string"
or even More Abscure
ProviderIncompatable Exception
Now after 20 mins of chasing my tail re different ver of EntityFramework 4.1 vs 4.2...blahblahblah
Look inside at the inner exception
A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible
DOH!!!!
Or a clean translation is that it cant find SQL or is offline or not running. SO check the power is on/Service running or as in my case 
Edit web.config & change back to Work SQL box
Hope you dont have this pain as the default errors in the EntityFramework 4.XX releases
Justin Oehlmann