Horn Blow instead of Cranking

I have a 2000 Chevy Venture LT. I went to start it and the horn honks instead of the car cranking. I tried my other key in the event that my key was bad and I got the same thing. I called the dealership and they told me my battery was probably bad. So I took the battery to see and the battery was fine and charged. I think it is stuck in securtiy mode. My security light only flashed one time in the message center. We have been trying unsuccessfully to start the van since saturday.