I would suspect she has an abscess in her hoof if she is that lame that suddenly.
If she is not better NOTICEABLY better and walking on it in 24-36 hours after getting bute, she needs a vet. If it is easily fixable, then the horse needs to have it fixed (like an abscess). If it is serious and not easily fixable.. well you have to make a decision especially with a 27 year old horse.
One decision that does need to be made is to not allow the animal to suffer in pain (and she is in pain.. she is not putting weight on a foot and is getting bute).
Allowing an animal to suffer pain is, IMO, inexcusable. You, your Dad and/or other forum members may not agree.
There is something about the outside of a horse that is good for the inside of a man. ~Winston Churchill
) Dinosaur Horse Trainer