Hello and welcome to JustAnswer!
What year, model, and engine do you have?
Have you tried your other key?
Ok. You're going to have to have it towed to a dealer for this one. You've got a bad wireless ignition node (WIN). This is the module that the key goes into, it reads the key, does the tire pressure monitoring and remote keyless entry functions. There have been a few problems with these here and there. For whatever reason yours isn't recognizing the keys that are programmed to it and will need to be replaced. If you are under 36,000 miles it should normally be covered under warranty.
You're welcome. Sorry to be the bearer of bad news. Good luck with your van!