2015 Odyssey completely unresponsive to key fob

I'm thinking that the car is not registering that the key belongs to the car. You might need to have the keys reprogramed or replace. Frist if it is a keyless push to start, try putting the key right on the start button to see if the car reads the key.

or you might new a new car battery. I would have someone check the car battery too.

I fixed my car! The immobilizer feature had been activated (a security cut off that essentially freezes the vehicle). For some reason the green key icon did not light up on my van. I have had this happen years ago on a Civic, but the icon came on. All of the information I saw online described how to reset the car using a physical key, but we have a push start van with no keyhole for the ignition (there is one in the door). Here is the fix: 1) On your key fob (with fresh batteries if needed as in my case): hold down the alarm button for 5s. In my case it almost instantly turned on and reactivated my car. So easy. 2) Reset sliding doors: My sliding doors got frozen slightly open during the immobilizer for some reason. To reset them, I slid the button over the driver's side controls to "off," then closed them manually. If the button is not in the "off" position you can't manually close the doors. Then I slid it back to "on" and could then use the controls.