To me, this would suggest that the blades are jammed in some way and are not freely rotating. You will need to check the mower deck is free from debris that is preventing the blades turning. With engine off, make sure you can fully rotate the blades by hand and if not, investigate further. The drive belts and pulleys will also need to be inspected.
Some machines, (I'm not sure about yours) have a safety switch within the drivers seat that senses if you are sitting on the machine before the blades will operate, might be worth checking if this applies to your machine and if so, check it's wiring or functionality. Your owners manual will provide details of any other safety interlocks that would need checking too.