The USAF needs to have a backup plan -- of some sort.
I am sure they do. However, if there is any kind of a prolonged stand down, ongoing air operations would be radically affected. We are stretched pretty thin, and especially with the ongoing budget mess. Tankers are an essential part of our force projection and the KC-135 is the bulk of the fleet.
Missile? Equipment failure? Pilot error? Catastrophic failure of some sort? It's too soon to tell yet.
It will take a few days to get the likely problem identified - unless there were current radio transmissions from the aircraft at the time describing some problem, and that might help. Otherwise, it is likely an on scene investigation of the wreckage and analysis of the Flight Data Recorder will be needed to nail down the exact cause. That will take a little time, although they can probably come up with a suspected cause pretty quickly once they can examine the wreckage.