Check the Terms and Conditions on your reservation. You should see this text.
"Unlike other car rental company or 3rd party websites, an Avis Prepay reservation allows you to make changes at any time, even multiple times, with no fee. Permissible changes include car type, options (GPS, protections, etc.), rental period dates/times and many changes to your rental location."
That just might be your "get out of jail free" card for this situation.
As long as you're somewhere with internet access, just log on and chance the reservation to a later date. That way, you can keep the money or possibly cancel it instead of being a no-show.