Stepmill stairmaster 5000PT can be programmed but then when you hit Start it does nothing.

Have you programmed as per StairMaster StepMill SM916 Owner Manual Page 24

Replacement Parts for Fitness Exercise Equipment at SPORTSMITH

Is this even after you begin stepping on the unit?

Start by checking the brushes in the alternator. The following link will take you to a troubleshooting video. This will help you resolve your issue.
Troubleshooting Video for No Resistance on Stairmaster Stepmill SM916

Stairmaster Stepper 7000PT Stepmill Parts Transmission SPORTSMITH net

My Stairmaster unit seems to work in manual mode, but resets when I select a program. What is wrong? It is possible that the console on your Stairmaster unit may not be configured for the appropriate machine. This can result in the console accepting and allowing a manual workout but not accepting or allowing a program. Stairmaster Units Works in Manual Mode but Not in Program

Check the following video out for some troubleshooting steps regarding this issue.

Stairmaster Stepper 7000PT Stepmill Parts Transmission SPORTSMITH net

Replacement Parts for Fitness Exercise Equipment at SPORTSMITH

There are a couple of different belts available for this unit. The following link will take you to the parts available for your model.

Stairmaster Stepper 7000PT Stepmill Parts Transmission SPORTSMITH net

console could be defective try to diagnose it by entering diagnostic codes from your manual
also check the mechanical parts inside , a hinge rod could have slipped out and has pinned the entire stairs

ensure that the console is programmed to stepmill ,as this console can be used on other machines

If you have a Stairmaster Stepmill 7000PT that has an older style console with the red LEDs and you want to upgrade to the newer, console with the blue LCD; the parts necessary for this upgrade include the console and the console cable.

Stairmaster Stepmill 7000PT Console Upgrade
C40 Console Repair Video for Stairmaster Steppers