One of the worst positions to sleep if you have a bad back and neck is on your stomach.
This position puts pressure on your joints and muscles, which can then irritate your nerves. It also forces your head to one side.
Your back is also arched if you lie on your stomach which is also bad for your back.
If you have to sleep this way then sleep with a very flat pillow and with your arms by your sides and not under the pillow.
I have always slept on my stomach and even if I start off on my side I wake up on my stomach. I do have a very, very flat pillow and when away in a strange bed I sleep without a pillow otherwise I would wake up with neck pain.
Sleeping on your back is the position to sleep in if you have a bad back and neck or otherwise the foetal position is ideal.