Probably water erosion has undermined the base over a long time, then more recently the slab eventually cracked under the stress hence it is now ‘settling’ relatively quickly.
One solution could be to first fix the underlying water erosion problem, and then inject mortar or polyeuethane under the base to float it back into alignment and then fix the crack using some epoxy crack sealer.
Another solution could be to knock the whole thing down and build a new garage/manshed in its place.
In any case you can try contacting the insurers just expect them to require insane amounts of paperwork and be really slow and convoluted and for it to probably cost you more than getting it rebuilt.