Why does my voice crack on high notes?

A crack is usually the vocal folds losing their grip as the mechanism changes between chest and head voice — often because the note was approached by pushing harder rather than by letting the folds thin. Sliding through the break slowly on a lip trill or a siren, quietly, teaches the transition better than attacking the note does. If it is a new problem rather than a familiar one, check whether you are tired, dehydrated, or unwell before treating it as technique.