[MCPE-36584] Minecraft deleted my world after a hard power off

Update on this issue by [Mojang] Mega_Spud (Jay Wells)

^12-Nov-18 - For those still having issues with their worlds not showing in the world list - In many cases the actual save data is still present on the device, so the advice from the developers in not to uninstall or delete the game data, as we are working on a fix that should enable the saves to be recognised and show in the list again correctly.

Steps:

Create world (created several days ago)
"Save & Quit" several times
Hard power off, by holding the XBox power button for 10 seconds (we had to because of controller issues)
Turn console back on
Open Minecraft

Previously created world not only doesn't have the most recent changes (as you'd expect after a hard power down), but the world is missing from the worlds list! My son put hours and hours of work into that world programming command blocks etc., and as far as we can see it is all gone.