I am developing a Sketchflow (Silverlight) project in Expression Blend 3. It has been working fine up until today, now I cannot run the project. Specifically in the Project menu the "Run Project" option is now greyed out (all the other options are fine). F5 also doesn't have any effect.

I've obviously messed up the code somewhere but I can't find any information on what could cause the "Run Project" option to be disabled. This would obviously help the troubleshooting. Does anybody know what controls this functionality?

I can build the project no problem. And if I package the project (so it runs outside Expression) this also works fine. It is just launching it from Expression that doesn't work.