I am having severe pain in the crook of my lt arm whenever i use it. the top of my arm is slightly numb as well as, top of my hand and thumb. ?

Nerve entrapment: This sounds like a case of nerve entrapment. The ulnar nerve is one of the three main nerves in your arm. It travels from your neck down into your hand, and can be constricted in several places along the way, such as beneath the collarbone or at the wrist. The most common place for compression of the nerve is behind the inside part of the elbow. This is called "cubital tunnel syndrome"

You need to be seen: I don't know what you mean by "crook" of your arm or even "top" of your arm. However, if you have pain in the arm as well as numbness, a nerve is being compressed somewhere. If the nerve is in your wrist or elbow, it can wait, but if the nerve is in your neck then you may have a disc collapse which could be an emergency. Severe pain is always a signal that something is going on, don't delay.