I had similar issue and problem was because of broken build paths to external libraries (e.g. Selenium server).
So first of all please open Build path (right click project -> Build Path -> Configure Build Path) and verify that all external libraries can be loaded successfully (there are messages like 'cannot load'). Most likely a library was renamed/moved while optimization or adding external tools like Ant.
Hope this helps and it will save your time goggling around :)
WBR,
Andrey
