Never ever use the cache lib for your own symbols. This lib is a special lib that must be considered part of the schematic file! only kicad itself should ever write to it.
You might need a lecture in proper lib management. Luckily i wrote one: Library management in KiCad version 5
This does however not really apply to the footprint side as there is no cache lib there. I have no idea why your footprints have vanished from the lib. Maybe you deleted the file or you moved it or renamed it. Could also be that there was a bug on saving or you do not have the right file permissions.
I really can not say more without you providing more info on how you made that lib and what you did since you last had footprints in it.