I hurt my neck two days ago and now my left are is numb and my thumb feels asleep. What can be the cause.?

Nerve bruising. You probably bruised your brachial plexus. Normally this should get better within 24 to 48 hrs. If there is no improvement or worsening you should see a neurologist or a neurosurgeon to get properly worked up. Sometimes an anti inflammatory medication can help. Since i don't know your medical history please go see your primary care doctor to be on the safe side.