One of the most annoying error messages generated by Eclipse has got to be:

The project was not built since the path is incomplete.
Cannot find the class file for
Fix the build path then try building this project.

The reason it is so annoying (to me at least) is it seems totally cryptic. In actual fact what has happend is the project that is generating the error has a broken reference to a JVM typically because you have upgraded it. You can choose which VM a project will use and you can also set it to use whichever VM the IDE is using. When you tell it to use the IDE VM you would expect it to symlink, in unix parlance, the IDE VM so that when the IDE VM changes the project updates as well. In actual fact Eclipse gives the project a direct reference to the VM. When that VM is removed the reference breaks. To fix the reference right click on the project, select preferences, and go to Java Build Path. In the Libraries tab select the VM (normally the top entry and it probably will have a warning triangle). Click edit and select workspace default from the dialog. Things should start working again.