Can a Cavity Fix itself?

No it can't, but some carious lessions do seem to peter out and arrest. The problem is that it is difficult for a dentist to be sure about whether this has happened without cleaning out the lesion.

Sometimes a long standing lesion has layers of blackened and dryed out decayed matter, but at the deapest part of the lesion there still seems to be some activity. This part of the lesion is wetter and a lighter chocolate color, that often seems to be associated with "active decay". Only by cleaning out the entire lesion and treating it can a dentist be sure that the lesion if arrested.