I have multiple Eclipse (3.52 \"Galileo\") ) workspaces so that I can keep related projects together in the same workspace. I switch between them with File > Switch Worksp
I think Bill LaPrise might be right in most cases, but not in mine. What I eventually had to do was go into the folder where I have all my projects and delete the .metadata for Eclipse (be aware that you'll have to re-import all the projects afterwards! Also all the environment settings you've set would also have to be redone)
After it was deleted I just imported the project again, and it worked.
This was on Eclipse 4.2.0 Juno, if you're wondering.