The mower blades won't turn when engaged. It looks like the cable that runs from the lever to engage the blades to the top of the mower deck is loose. I have a Craftsman 42" riding mower

Hi,
The arm that pivots to engage the blades, where the cable attaches, has a bushing that it pivots at. Dirt, rust and/or corrosion can cause it to stick. A bolt holds it together and passes through the point of pivot. Disassembly and cleaning this bushing is called for. It could be a bushing beneath a washer or a step bushing.
Peace,
Clarke