A plant suddenly dies, the leaves turn brown but remain on the plant.
Cause
There has been some poison of some kind. This could be animals weeing on a plant, someone pouring old cleaning liquid, spilling drinks drunk at a party, etc.
Harm
The toxic liquid is taken up in the plant and kills the whole plant or parts instantly.
Treatment
The only thing to do is to water the plant copiously to help dilute the poison. However, usually the damage has already been done and this only has limited success.