Why it won’t start after replacing deck belt

According to CoiF on LawnWorld, "Safety switches are located on the Brake Pedal (must be fully depressed); Seat (must be occupied), Deck/Blade/PTO (blades must not be engaged), Transmission (prevents operation when blades engaged and trans in reverse).

Since you just worked on the deck belt, I'd look to the Deck/Blade safety switch first. You may have changed the position of the switch or disconnected the molex connector or damaged the wiring. The switch will be located near the "arm" that is moved by the blade engagement cable to tension the blade belt for engagement. When moved to the blades engaged position, the plunger/button on the switch is fully depressed. When Blades are disengaged, that arm moves away, and allows the plunger/button to extend back out. The plunger may be stuck. The connector may have become loosened or dislodged. The wire may have been damaged/crimped/severed."