Need help with a John Deere riding mower

I have a John Deere 100 series riding mower. I have had it for about 10 years and never had a problem with it other than replacing the battery. I was just outside cutting the grass and the blades disengaged and stopped turning. There is a yellow knob to the right of the steering wheel that controls the drive belt. I pushed the knob in and pulled it back out to engage the blades and they started turning for about 3 seconds and disengaged again. Now it will not engage at all. The belt is not broken or off the pulleys. I am a do it yourselfer, so I am going to start troubleshooting but I thought I would post on here and see if someone has experienced this and what the fix was.

Need help with a John Deere riding mower