First and foremost stop your exercises immediately. Take rest, hot water fomentation, analgesics, muscle relaxants. Do visit a doctor for confirmation of your diagnosis. When you are 100℅ OK only then resume your exercises under guidance of a qualified physical trainer for correct posture during exercise. Very often injuries in the gym stay for a long time if exercise is done incorrectly.

Hi there...I can understand your concern. Sudden onset of neck pain and headache could be due to some muscle spasms or sprain in your neck ( could be due to excersising in the gym), any cervical lympadenitis, disc prolapse. You will need detailed clinical examination. So kindly consult a orthopedic for further evaluation.