Oh dear - I really feel for you! I had one cave in not that long ago ..... I reckon I've had about 5 'go on me' in the last 5 years. Mine were just age, I'm afraid (having a few years on you!! ;) ) - the blood supply to the teeth is less as we get older & they become brittle & crack/break. They are pretty good at doing the resin 'fix ups' these days! I hate that sharp feel tho - you just can't stop 'playing with it' with your tongue!
Have you been able to get a quick appointment with your dentist? I hope so!