Three years ago I had my C45 neck fused. Since that time the C56 level has become unstable. Is it possible there was something wrong at C56 back then and they missed it? I could have had them both done at the same time.
Many studies have shown that spinal fusion puts increased stress on the adjacent level (above or below). It's not uncommon for problems to occur later at the next level.

About two to three per cent of the patients who have cervical (neck) fusion develop new symptoms of neck and/or arm pain each year. Disc degeneration is the most common cause of the symptoms. Spinal joint arthritis and joint instability can also lead to similar problems.

If there were changes at the C56 level at the time of your first operation, the X-rays would have shown them. Usually it's the case that X-rays appear normal before surgery but changes occur after the fusion.