What is wrong with my neck? I woke up and one side is tighter than the other.

Pain in the neck. Likely that you slept with your neck crooked/tilted in one direction too long and thus having some muscle spasm, hence tighter on one side vs the other. Warm compresses, gentle stretching, and use of muscle rubs/bengay, motrin/aleve etc. May help. If not better in couple of days, consult your doc. Good luck.