Backache that prevents one from bending can be due to (1) stiff back muscles, and (2) spinal disc herniation.
(1) Stiff back muscles are caused directly by a sunken and uneven bed and mattress. It can also be the direct result of a bad chair, sleeping in a stiff posture, being forced to stay in or maintain an upright and erect posture for long periods, etc.
(2) A herniated disc of the spinal cord may produce a constant and dull backache that causes the back muscles to stiffen and tighten, thus preventing one from bending.
Take a hot shower with the back towards the shower head, then apply a generous amount of vapor rub or muscle pain relief gel on the entire back to relax the back muscles. Correcting a disc herniation by acupressure or physical therapy exercise may also be useful to get rid of the backache.