May have been hit by a car or snagged jumping a barrier. Deer do recover from collisions - even severe ones - and broken or fractured bones can heal in time. Nothing to be done if the deer can still walk.

A herd in your yard would likely protect an injured member if you tried to approach causing you some pretty good trauma, but in the wild a deer that can't graze and feed itself will just starve to death.

This past record breaking winter culled our herd - haven't had one in the yard yet this season and I live just off the edge of an in city nature preserve. Five feet plus of snow on the ground and weeks of sustained temps in the minus 30-40o range took care of our neighborhood grazers. I'm seeing plants actually bloom for the first time in a handful of years.