Usually that indicates that part of your game files have become corrupt or are incompatible with more recently updated versions of those files.
To fix this:
1.) Click ---> Optimize Your WGT Experience
2.) Follow the instructions to clear your Flash & Browser caches. They are located near the bottom of that post. Make sure that you clear both caches.
3.) Close your browser, restart it and try to play again.
That should take care of the problem. Be aware that clearing your Flash cache will reset your in-game options, so if you've changed the graphics quality or turned off reflections you'll need to re-set those options again.