my mattress

My mattress sinks down on all sides. It makes an awful noise when sitting or lying down. I can push down on the foot of the mattress and it pops up at the head of the bed., and vice versa. I have had spinal surgery 14 years ago. I had my old mattress for 10 years and never woke up with a sore neck or stiff back. Can you tell me what is wrong with my mattress and how I might fix it, even if it means buying a new mattress.
Thank You,
Linda