I did something to my tailbone yesterday and now it really hurts. It's either broken, cracked or is bruised. Should I put ice or heat on it?

Ice. As a rule of thumb, for an acute injury, it is best to put ice on the area for no more than 20 minutes at a time each hour, for the first 24-72 hours. Putting it on for longer than 20 minutes could result in a superficial frostbite.