I have sudden weakness and pain in my left hand. My right is fine, but grasping with left is weak and a bit painfull why is this?

Arthritis, maybe. Sounds like this could be arthritis. Hand surgeons are quite good at helping people with hand and wrist arthritis. See one for an exam and an x-ray. I bet you'll be happy you did.
See your MD. There could be some nerve compression or compression of the spinal cord creating your weakness. Anytime a patient experiences weakness, they need further evaluation right away. Call your family md, let him evaluate you and order the proper tests. He will refer you to a specialist if necessary.