I, a long time ago had a Yucca that grew too large for the house, I put it outside and the top suffered badly, I cut it off and after a while (cant remember now how long) I got more shoots growing from the sides, it ended up at my Sons garden it survived untill he moved it again and it died.
So if all your leaves drop dont dispair ! with patience it will probably come again.

Sue, are you sure your plants are Yuccas and not Cordyline? Only your description sounds exactly like what's happening to vast numbers of Cordyline round the country at the moment, as a result of the harsh winter. If they are Cordyline (google to see) check the trunks for signs of rot - if you find any, just keep a close eye on it and if it gets to the point where the tree is falling over because of it, you'll have to cut it back now beyond that point. Otherwise, just wait - if any rot present is worse by April, then cut back beyond it then, not now, if you want to try to conserve the plants. If there's no rot, then leave them alone - they may, by June, sprout new growth either up the trunks, or from the base off the roots, and when they do, you can cut back to wherever the new growth is.