Could be several thing have you got more info/pic please? such as are the roots mushy, dug at, or infested?

Vine weavil?

Look as if cabbage root fly has eaten the roots to me.

If it is cabbage root fly what is the treatment?

If its cabbage root fly there should be evidence of white grubs around the root, I think it looks like club root

Club root? All of my plants were healthy until recently and all at once they have keeled over and snapped at ground level I thought it may be an under ground type catapillar (not sure what they are called) What is club root and how is it caused? thanks for answering

Club root is a fungus in the soil it likes warm moist acid soils, the spores enter the root hairs and causes an abnormal swelling on the root which restrict water and food intake. Liming the soil can reduce acidity, and dipping the new transplants in a fungicide can help (Flowers of Sulphur). You must burn the affected plants.

I dont think it is club root as the roots have been totally destroyed and I have not observed any swelling or abnormalities. The root balls were completely healthy when they were planted out earlier this year and the plants were very strong and grew as expected until quite recently. I do not see any evidence of pest infestation lava, eggs or otherwise...I do not see evidence of fungi or rot I have to admit it has me stumped (excuse the pun)
The only thing I can think of that it may be is the underground type catapillar that feeds off the roots of healthy plants until they literally fall over which is what is happening. I am open to other suggestions because I am at a total loss right now. Thank you again for your input...
I need a treatment soon or I could lose my entire crop.

The only thing I found was a Cock Chafer Grub that lives underground and it does eat seedlings and young plant roots, give it a google!

We used to use collars round the brassicas to prevent the fly getting down and laying its grubs... But as Dido says if you have that particular grub you will see it in the roots munching away. There is good info on how to deal with on this site:
http://www.allotment.org.uk/vegetable/brassicas/cabbage-root-fly.php

The damage caused by club root and root fly looks very similar so it could be club root as Dido suggested.