Hi,
I also had the same problem after my retrosigmoid surgery, which aggrevated a previous neck/shoulder problem that was attributed to poor positioning while using a computer. It was very painful, I couldn't sleep, same as what you describe. Unfortunately this is a common side effect from retrosigmoid, because they cut through the sternocleidomastoid muscle.
I will second the pillow suggestion, I bought an expensive orthopedic pillow which supported my neck and it helped a lot. I was taking Tylenol for body pain (with the muscle relaxer) for quite some time, as well as Aleve, and used warm compresses for the neck. However I also had nerve pain on the back of the head, which needs the opposite: ice packs, applied every 5-10 minutes. Sometimes in the evenings, I ended up having both the warm compress on my neck and the ice pack on the back of my head!
I also did physiotherapy, and that helped too, especially because my motion was so restricted (I couldn't lift my arm all the way, and had trouble turning my head backwards). I bought a new workstation for my computer and learned how to use the mouse with my other hand.
Indeed this problem is awful and it makes it hard to get some good rest and recover. But it does get better. Even though I still feel that my neck is sore from time to time, things are pretty much back to normal.
Marianna