Sick Orchid

Alice, I can't be sure from the photos, but it looks to me like you have either fungal or bacterial rot setting in at the base of the plant. Unfortunately, either one is hard to eradicate.

if it were my plant, i would probably treat it for fungal rot by applying sulfur powder to the affected area, and hope for the best. You probably don't have sulfur sitting around (though it is sold at garden centers and is fairly cheap) so you could try another fungicide. Even Lysol might help, as it has some anti fungal properties. Some growers swear by hydrogen peroxide, but i haven't had much luck with it.

But i suspect that this plant isn't going to survive, so be prepared for it to go downhill fast.

 

In the first photo - if there was a problem at all - it was likely just an infection caused by water standing in the folds of the leaves. That can be avoided by watering early in the day so it has time to dry before nightfall. However, it may have been nothing more than the natural process of the plant to resorb nutrients and drop older leaves.

Unfortunately, that last photo shows that you have completely removed the apical meristem (growth front) of the plant and have likely killed it. If there is enough vegetative mass left, it may form a new growth front somewhere, but that looks to be pretty severe surgery.

In the future, if you see what appears to be a surface infection like that, use a cotton swab to dab a small amount of cinnamon powder on the wound, avoiding getting it on the roots as much as possible. Cinnamon is a decent fungicide and bactericide, and also a desiccant to dry up the wound.