I had the same problem and solved it by saving the razor file (Ctrl + S). Once I did this I could use the Enter, Backspace and Navigation keys again.
I believe that Alt + Enter may also work.
I don’t often get into this state and the above solution does not solve the underlying problem. I believe that it may have something to do with ReSharper but have not figured out if this assumption is true or not.
There are a number of pages that discuss this type of problem:
Link 1
Link 2
Link 3