I woke up and the left side of my neck hurts, what to do?

Pinched Nerve/Muscle: Most times back pain that is sudden onset will resolve. You should use anti-inflammatory meds like ibuprofen, aleve around the clock and even tylenol (acetaminophen) to help. Stretch and Ice the area. If this does not resolve you should see your doctor for other options. If it still does not resolve, i would recommend to see a spine specialist for a full evaluation and testing. You may need to get an MRI.