Depends what kind of bad thought is it? If it's that type of intrusive thoughts then you should just talk to someone via chat or phone just to keep your mind busy. But if it's more dark and much more serious than that, you should take a deep breath and try to be mindful of your surrounding. Try to recall what happens today, all the small things. Maybe try to take a bath or go for a walk in a green park where you're surrounded with nature might helps.