I had a cavity filled in the back of my mouth when i was younger. A few weeks ago, a little of the tooth chipped off underneath the cavity. Now, after eating some toast i notice an empty slot where the cavity had been

well i was just wondering.. for some reason there isn't much tooth left there. what do you think a dentist would have to do? It's a pretty big hole and it's soft like gums.
also, is it ok to have swallowed a filling? one of those silver/grey ones..