There should be a safety switch, but not under the mower deck..Grass and moisture would Knock it out quickly..
It could be ON top the mower deck, or near the lever that engages the blades...
Or near the blade clutch if elctronic..
There should be one under the seat, and one on the brake or linkage..
Also possible one near the transmisison to insure starting in neutral..
An OBstruction could be under the deck, but usually would not stop it from starting..
Follow electrical wires to find safety switches.. Inspect them while you are following the wires..
Check to make sure stitches are being engaged.. ANd they are in good condition..
Check them with a meter to see if they do anything when engaged..
Is the starter turning over, does it just click..
Is it ONE click or several clicks in sucession??