When the system (or Kate) crashes, all editor content is lost. It would be great if Kate supported crash recovery, like most editors do, even console based ones, by continuously saving a hidden backup file on the fly in the background.
These automatic backups are called "Swap Files" in Kate. Please check options in Configure Kate > Open/Save > Advanced.
Did comment #1 help?
thanks @Christoph_Feck didn't remember that one, seems I've overlooked the swap file then, the option is active.