I can't contemplate a scenario where all coins in the world just "disappear"...

However, it really isn't that far fetched to imagine a scenario where at least part of the world will not have electricity to power computers or networks (North Korea EMP threat for one).

Although not a full on disappearance of all one's coins, it would essentially render them useless and out of reach as long as an electrical interruption continued.

We will also likely see more large scale breaches as time goes on and attackers find more innovative ways to access systems that were previously impervious to them. We've seen major exchanges and cloud mining companies compromised, if you were a victim of those situations, your coins disappeared.