Second question first. There is a safety switch on the brake pedal and also one on the deck engagement. Also there could be an issue with the starter solenoid being bad. So may need to get out the volt meter and do some checking. Further assistance there if needed.
And for your first question. take a look under the mower up under the frame to make sure the two belts are in good shape and not came off there pulleys. And then from there I would try starting it and while leaving the direction control in N move the speed control lever to a high speed setting and let the clutch/brake out. And then pull down the brake, and then move the speed control to a slower speed, put the mower in either F or R and then see if it will move.