Why won't my car start?

I have somebody come out and look at my car it's not the battery it's not the
starter it's not the alternator is not a fuse the radio comes on when it wants to
its that's a whole different subject but it does come on the lights work the the
door dings when you open it all that working the only thing I can think of is
possibly maybe anti-theft something to do with that please any insight would be
very helpful thank you