How to save this cactus please

Hello, I have 2 cacti the same, think they're Echinopsis. Recently one has become very ill looking. It's become pale and shriveled.

Last week I took it from the pot and soaked it in a dish of water, but it hasn't improved. Very little root on it when I took it out.

Any suggestions please on how I can save it.
I've had cacti for many years but not seen this happen before.

 

They probably wont be doing much this time of year so I'd be careful with the soaking, just keep them barely moist and not near a heat source, possibly on a window sill until around mid march, then you can try to bring the wrinkled one back to life, the other one looks fine to me.

 

Yes, the other is fine, nice and healthy. This one has been going downhill since spring / summer shrivelling more and more

 

It sounds as if the problem is with the roots. Have you noticed vine weevil grubs in the soil ?
Another reason could be that it's been kept too damp, not enough drainage maybe and the roots have rotted ?