I have dowagers hump, is it safe for me to sleep on my back?

Yes. I cannot think of any problems with this although it may not be very comfortable for you. You will not break anything if you sleep on your back if that is what you are asking. If you have a large hump, this may cause your neck to sink to an excessive degree on the pillow and give you neck pain. You should make sure that your neck is level with your chest of perhaps even a little higher.