99 rodeo shift knob button will not push in

I have a 1999 rodeo ls v6 auto trans. Tonight I went out to leave work and when I started the engine and tried to shift into gear the shift knob would not push in. I thought maybe it was because of the cold and let it stay running for approximately 5 minutes until the engine temp showed at normal. This did not help. I engaged the brake all the way down to the floor and still no go. Has anyone had this problem or know what may be causing it. There is 85k on the car. Please help I don't know much about cars so I don't want to take it into a shop and get screwed over.

First make sure the shifter is fully in park and try again. When you press the brake pedal you should hear a clicking sound. If not it could be the sensor is gone. If you have a manual you can get instructions on how to dismantle the shifter console to get a better look at it

Thanks. Depressing the brake pedal no longer makes the clicking sound so this is most likely the cause. Does anybody know how much this sensor would cost to have replaced?