My posterior deltoid muscle won't work two weeks after a neck injury the muscle would just stop working and give out when used. Not much pain but some?

Get Exam. Your question infer that you have been able to isolate your lack of function to a specific part of the deltoid. My question to you is how? Even though you reference a neck injury, you provide no other information. You may have an injury to a nerve or a rupture of the muscle. Please see your pcp for exam & referral to an orthopedist.
C4/5 Disc. You need to rule out a c4/5 disc herniation. Please have a screening MRI cervical spine and consult a spine surgeon ortho or neuro.
Perhaps nerve root . A neck injury could cause a disc herniation and a c-5 root problem causing this, but a direct injury to the axillary nerve might cause a problem, and perhaps your issue is a rotator cuff syndrome in the shoulder. Suggest starting with an orthopedic surgeon.