This means that the application is unable to load the EDMX.
Your connection string might be wrong. you might not have changed it, but if you have changed other things (say, the name of an assembly), it could still be wrong.
You might be using a post-compile task to embed the EDMX in the assembly, which is no longer working for some reason.
- Delete your connection string, EDMX and all references of EDMX
- Re-add your EDMX. Add its reference in the projects(s)
- Build and run your solution.