What to do if i had a cervical fusion a few years ago just a few days ago my neck started to hurt on the left side with a headache i wake up with this?

Hard to tell: If you really had a successful fusion a few years ago then I would be less worried about the fusion than other things that affect your neck - sprains, a new disc herniation etc. If this is a really bad headache a vascular problem could be considered. If this does not get better after a few days or if you have other symptoms I would see your doctor

Time: Hopefully, you just have a cervical strain. If the symptoms persist, them see your Dr. for an evaluation. Good luck.