There are several ways to become immobile, such as having two injured legs or being knocked over by a Critical Hit or explosive. Having inventory weight exceed the Carry Weight capacity can also cause immobility.
The cure for immobility depends on the cause. If one has two injured legs, using the Doctor skill to heal one of them will cure this condition. If one was knocked over by a strong hit, one will stand up after a period of time. If overencumberance is the cause, dropping some items from inventory or passing them to another squad-mate should help.