Yes, you can lay on your back after surgery. To reduce pain after a spinal procedure it is typical for a patient to lay on their side or, believe it or not, on their back. Although, it is suggested to keep your knees bent and a pillow tucked between your legs while sleeping in either position. However, due to a increased pressure on the bones in your back and the increased pressure on your disc space, sleeping on your stomach should be avoided.
Always, ask your medical professional which sleep position is best for your specific procedure though.