rhurarb is a very hardy plant that rarely gives trouble,they will even stand shade, the only reason,s i can think of is 1 crown,s are under ground level, 2 waterlogged, they need drainage , 3 starved of food, as they are hungry plants, 4 if they were planted last year and you took to much of the plant

when you say rotted off do you mean the stalks from last year have rotted off? if yes then this is normal. Do you have any pink/green knobbly bits in the centre? if yes this is going to be the new growth.
if the whole thing is rotten then the root growth last year may have been eaten away and then the centre will also die off.

You can add replies such as saying thank you in the comments box. That way others know what you are referring to. It is a bit like having a conversation in writing. Do not forget to click the Add comment box or your message will not be sent. You will know it has been sent if you get a message at the top saying Thank you for your reply.