What would cause a loss of range of motion in a hip? I am unable to abduct left hip. Upper leg muscles feel weak, leg feels "stuck" when I abduct.

Any groin pain? Any pain or limping with weight bearing/walking? Morning stiffness? If so consider hip joint problem. Consider labrum too. If groin muscles are sore perhaps a muscle strain. Bursitis of the lateral hip or gluteus medius/minimus muscle problem perhaps but you would have soreness of the lateral hip. Nerve issue can also cause weakness - look for numbness or tingling in other parts of the leg.