When you lose muscle, is it gone forever? is there any way to restore (or repair and/or rebuild) lost muscle?

Usually, yes: This is a complex and great question that somewhat depends on the mechanism of injury and what muscle you are talking about. As a young person the ability to achieve repair and rebuilding are significant. Working with a physical therapist, augmenting nutrition and exercising can assist tremendously in this regard. Tell me more about what happened so that i can provide a clearer answer.

Muscle adapts: Muscle adapts to its level of activity. If its not getting used much (lack of exercise) it will lose its tone, size and shape. Adding moderate amount of stress to muscles through exercise forces it to adapt to the stress, resulting in firmer tone, more appealing shape, and larger size and capable of more power and strength. Resistance training should be part of any exercise routine.