The jawbone starts shrinking weakening neighbouring teeth until they collapse.
When you lose a tooth, it disturbs the interplay between teeth and bone. Because gum and bone are no longer stimulated well enough due to the missing tooth, the jaw bone starts shrinking and your gum pulls back. This can weaken neighbouring teeth until they collapse. Teeth in the opposite jaw can then start growing into the gap.
Missing teeth and a reduced jaw bone can make your face look older and wrinkly, and for your cheeks to become hollow and saggy, because they can no longer fully stabilise your lips and cheeks from the inside.