Will not start and theft light is flashing

It is a shot in the dark but try having the dealership test the key. If it is a chipped key, which it probably is, then the chip could have been damaged/demagnetized. This would keep the car from starting and cause that light to flash, on and off.

That's a good idea. It could be a neutral safety switch/range switch gone bad or out of adjustment. Often times you can put it in neutral and it'll start, so definitely try that.