From: This site
Cryptocoryne species may suddenly have a complete meltdown, where all or most of their leaves simply disintegrate. The exact causes and circumstances of this has never been fully established. It has even been referred to as a disease.
It is known that various environmental conditions can trigger it. Sudden changes in temperature, water quality, nitrate or ammonia levels and transplant shock all may set off this condition. It has been observed happening in natural habitats during seasonal changes and periods of reduced rainfall. It could very well be a defense mechanism. If left undisturbed in stable conditions, the rootstock, or rhizome, will usually totally regrow new leaves within a few weeks to a few months.
The biggest mistake hobbyists make when this malaise strikes is thinking their crypts are dead and then preceding to throw out what’s left of the plants