onions rotting

Its sounds like Onion White Rot, it's a white fungus growing on the roots causing them to rot away. It makes the onions inedible and they won't store. It is very serious and means you can't grow on that patch again for several years. I do hope I'm wrong.

It could also be onion fly, you should be able to see the small maggots when you pull up the onions although you should have noticed the leaves yellowing before that.

 

With White Rot it depends on the stage of the disease, if you catch it early the leaves look OK.

At this time of year onion tops are beginning to fall over and soon will be yellowing and dying off naturally, just to confuse things.