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.