Welcome to the Bunnings Workshop community @Davina12-37scot. It's great to have you join us and many thanks for starting a discussion.
I'm sorry to hear about your experience with the weedkiller and your lawn dying.
If you could give provide members with a few more details I trust that will be helpful in offering solutions. Perhaps if you took some pictures of the affected areas it would be helpful in us understanding how much damage the weed killer has done. If you have killed your lawn completely and it is now completely brown and dead, there is not much you can do for it so you would be looking at establishing a new lawn.