I've found a way to solve this problem without changing any code or running commands like "Convert to Web Application" - and it's simple too!
What I found was that restarting Visual Studio often solves the problem, but sometimes it doesn't. In those cases, if you close Visual Studio and then delete all content in the "obj" directory for the web project before you open it again, it has always worked for me.
(when started again you just add a space and remove it again and then hit save to have the designer file properly regenerated)