I lost safety key for sportcraft treadmill tx335, where can I get one?

Yes, that is literally the sole purpose for the neutral safety switch. To prevent the vehicle from starting in gear. If the neutral safety switch is malfunctioning, there is a very good change that the vehicle will not crank.

Neutral safety switch on the transmission.

Probably out of fuel

You may be able to hold the original key close to the key lock and try starting the truck with a valet key that does not have a chip.
The sensor for the chip is close to the cylinder.

Turn key,listen to the whine of the fuel pump priming,start bike. If you don't get the sound of the pump priming either it's not getting power or it's dead, I don't think it has a inertia safety switch.
OR
the crank sensor is the only sensor that, if faulty, will not allow a bike to run. However, even with a bad CS the bike should still 'prime' when you turn ignition on and switch to the run position.
( if it doesn't, I'd look for power to the pump)

I need a key for my healthmate treadmill

If the car does not have a security system, I would check for voltage at the starter from the battery and from the key switch.
The power from the key switch goes thru the neutral safety switch before getting to the starter.

TRY TO START THE VEHICLE IN NEUTRAL IF IT STARTS YOUR NEUTRAL SAFETY SWITCH HAS MALFUNCTION, OTHERWISE ITS MORE THAN LIKELY THE STARTER, TRY BOTH KEYS ,THE COMPUTER COULD HAVE LOST THE CODE IN THE KEY

Have the key and ignition checked! I am assuming that the key has a security chip on it, Right? If so, re-program key at dealership, and if that alone doesn't work, have them check both the key and column.