In Transformers 4, Optimus Prime was badly damaged in the battle and could not transform or start.
In order to restore Optimus Prime, the Autobots found an abandoned replica of Optimus Prime and repaired and rebuilt it. They replaced and repaired the damaged parts of Optimus Prime and upgraded some new skills and equipment for him. Finally, the Autobots successfully repaired Optimus Prime and let him return to the battle to protect the earth.