I've read that rhubarb prefers a well-drained soil with organic matter. I planted my roots in loamy sand mixed with compost and manure, it retains moisture but also drains freely.
Clay soil may possibly retain too much moisture, which could result in rot then browning as you describe. You could try carefully digging up a crown and seeing if there is any sign of life.
I'm a novice at growing rhubarb, just repeating what I did, based on what I read and advice I received.
Good luck!
