I have a problem with my gear shift?

I have a 2006 Pontiac G6. Today I went to put it in park, but the car didn't stop. I thought I didn't move it properly, but when I looked down it was in park. I had to hold my foot on the brake so that the car wouldn't move. When I went to turn the car off I had problems stopping again. I held the brake down, put it in park, and turned it off. I tried to turn the car back on and it wouldn't start. Everything comes on it the car, but it just won't start.