The outcome of the regeneration of peripheral nerve following injury is often poor - the ability to control muscles may be impaired and sensation is rarely normal.
Peripheral nerves consist of many nerve cell processes in parallel in close association with a second cell type, Schwann cells. (A picture may help to appreciate the structure.) If a nerve cell is seriously injured, the peripheral portion of the cell dies, and hence becomes disconnected from its "target", e.g. the muscle cells it may have controlled. For the function to be restored, the proximal surviving portion of the nerve cell must grow back to the target. In humans, this growth may be over large fractions of a metre, and at best proceeds at only 1mm per day.