How soon do you need to fix a filling that fell out? Is it bad for your tooth if you leave it alone?

Answer 1

Leave it alone. It will fall out by itself.
More decay is in your future.

Answer 2

It is important to have that cavity repaired as soon as possible. It fell out for a reason--either there is decay, or the tooth or filling broke away. In any case, bacteria can now get into your tooth and cause infection. Leaving it alone may not hurt now, but it will later, and you may lose the tooth!