all the way around the edge (the middle is still green).
What did you feed it with and did you follow the instructions to the letter?
Did you water over the leaves?
Whats in my mind is it has been frosted which in itself will do no harm to the plant just the leaf in question.
I realised the plant was maybe too low in the soil, so I have put a spade into the ground next to it and lifted it just a fraction.
Then again this might be your problem!! You might have broken the tap root and any new growth has died back, particularly if it is a young plant.
The plant should recover but you won't get any edible stuff till later in the year.
It looks to my very untrained eye as if the plant has too much or too little of something!
Yes to much tlc I think ;D
Rhubarb is a garden toughy so I don't think you will have killed it.
My advice now is just leave it to its own resources.
My rhubarb has been in the same spot for as long as I can remember it gets no particular treatment at all. It only gets fed if I am in the area feeding something else if not it gets nothing.
This is how it should look in early season;
and this is later when it has matured;