problems with 300 5.7l 06 chrysler

I had recently walked into a gas station and my 06 chrysler 300 was working fine when I went in and I come out and it didn't recognize the key fob.So I replaced key fob and then programmed it to the car and got it out of anti theft mode then and tried driving it and now it won't go over 35mph.Which I'm guessing from what it looks like is called limp mode.How do I fix this I've seen multiple supposed ways but the main one I think is replacing the cars computer and reprogramming used computer to it.Is this so? Or what should I do.:banghead::banghead: