Nymphaea lost all leaves


My Nymphaea (sp Swaziland, I think) has lost all its leaves over a 
period of several weeks. The leaves do not decay whatsoever but just 
break off at the gravel. What is wrong with it? I tried to remove it but 
found somekind of growth as big as a thumb under the gravel. What should 
I do? let it stay in the gravel? Would it grow leaves again? Thanks for 
any advice.