Why can't i straighten my finger after a punching injury if there is no fracture? it stays bent and knuckle seems pushed back. x-ray read normal.

Be seen : I would certainly be seen in follow up for a specific diagnosis. A tendon rupture is just one example of an injury that will not show up in X-ray but will have symptoms consistent with what you are describing. Follow up with an orthopedic surgeon.

You may have: an injury to the extensor tendon("mallet finger"). You may require splinting to avoid deformity and loss of function, so you need to see your doc or an orthopedic surgeon for a complete evaluation.