I wonder is there is a way to cancel the previous command you imputed ( just because my /fill command didn't go so well )
Unfortunately there is no way to do that. Personally I ensure to backup my world before using /fill or /clone in the event something goes wrong, but that's pretty much the only solution to fixing such problems.
Unfortunately there is no way right now, but if you have backup then you can do something. i searched some forum answer then everyone said about the backup thing. i found something like adding undo/redo on 1.14 version here.