idk what this is

click ctrl-z to undo.  you can repeatedly undo steps.

that would solve the problem IF you did it without changing timelines.  otherwise, you can look for a saved version if you have auto-saved enabled or you should return to a previous pre-problematic version (if you have one).

you probably enabled a filter or changed the blend mode.  you may be able to fix that without returning to a previous verson or undoing.

and from now on, save your fla's with an increasing version number appended so this and many other issues don't happen in the future.