Try reinstalling roblox. Follow these steps to reinstall roblox:
1. Open Control Panel(to open Control Panel on Windows 7 or bellow use the start menu. On Windows 8 put your cursor at the top right hand corner then move your mouse down to Settings > Control Panel
2. Click Add/Remove programs, uninstall a program, or something similar.
3. Scroll down until you see Roblox Player For[your computer username]
4. Double click the Roblox Player For[your computer username]
5. A window should pop up that says Uninstalling Roblox. Wait for it to finish.
6. Go to www.roblox.com/install and click the big green button.
7. Click run/open etc
Now try to play a game and see if it works.