How can I save my Echinocactus (golden barrel)?

It doesn't sound very hopeful, it may have rotted off inside. Try to see if you can pick it off the compost (wrap thick paper round it as a collar to save pricking yourself), if it comes away without roots it's a gonner. If there is resistance its not rotted completely. Has it been overwatered?

I agree with vounteer, sounds like its rotting off. They really need to be kept bone dry during the winter. Has it been to cold ?

Thank you for your replies. The cactus is holding by the roots still. The cactus is in the house, I will move it to our warmest bedroom. I have bearly watered it for the past few months. Other than the constant water it is leaking, it looks very green and healthy with the yellow like 'fluff' all over the top.
How do I get rid of the flies? I am worried about spraying it. I feel so sad that is may die.
Thank you.

They may be coming out of the compost. Use Ultimate bug killer from Provado, follow the instructions on the packet. Move it to the sink or somewhere that you can wipe off any drifted spray. There are no predators indoors so spraying is really the only way to get rid of them. Or you could put a sticky fly paper next to it to catch them when they fly up.

Thank you Volunteer. I will try this and keep my fingers crossed!