First step, call to get into your dentist at the earliest time and to consult with them on any specific advice for your situation. In the meanwhile, here are a couple of short-term steps you can take. There are dental cements available over-the-counter that you may be able to use a temporary stop gap. A piece of sugar-free gum can also be applied into the cavity until you see your dentist. Make sure the gum is sugarless as even a little sugar put into the cavity will hurt!