I recently disassembled my rear shock from the bike for a spring replacement. The shock and the preload adjuster are two separate things (I'm assuming you have the OEM shock). The preload adjuster is a knob connected to a cylindrical ring with fluid inside that sits on top of the rear shock. Right above the spring. It almost sounds like you have two problems: a seized shock, and a broken preload adjuster.
1. Can you see if the preload ring is fully extended at the moment? If you can see the inner ring out about the thickness of you ring finger, it's fully extended.
*Stuck/seized preload ring on max tension setting.
2. If you turn the knob, does it go very easy?
*No fluid inside or broken knob. But you would expect the preload ring to be fully retracted.
3. If you turn the knob, can you see any movement of the preload ring at all? It goes pretty slow, but it should move.
Does the shock not compress at all, not even a little? If you put it on the center stand and push it off, the rear should compress 3cm / 1.2inch.
