That appears to be 'Cryptocoryne melt'. It typically happens to me (but not always) whenever I drastically change the water parameters in which my crypts are growing. If I change tanks, or breakdown and then replant in a cleaned tank, or sometimes with just a major (well over 50%) water change it can happen. Sometimes I lose just a few of the older leaves, sometimes I lose them all. I think it is the cyptocoryne's way of adapting in nature to changing water conditions and water height during the 'dry' and 'wet' seasons in the tropics.
The good news is cyptocorynes have rhizomes. As long as the rhizome and roots are healthy the plant will start putting out new leaves typically within a couple of weeks.