If you lose a filling, call to get in to a dentist at the soonest available time. In the meanwhile, there are a couple of short-term remedies you can try. Inquire at your pharmacy, and pick up some over-the-counter dental cements available to use as a temporary stop gap.
For an even shorter term remedy, you may also use sugar-free gum to cover the cavity. Be sure it is sugar free gum, because any sugar entering the cavity can cause severe pain.
Cavities left untreated can develop into more serious problems, including loss of the tooth.