My first guess is that you might have loosened the electrical wires at the PTO. This would be a wiring connector near the front pulley, which is directly under the engine (not one of the pulleys on the mower deck.)
Sit in the driver's seat, turn the key to the "on" position, but do not start the engine. Now pull the mower engage knob. You should hear a "click" from under the front of the machine. If not, you've likely got an electrical disconnect somewhere.
javert
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