lost widgets section

Hi. On one of my sites I have lost the entire widget section. The available list of widgets is there but I cannot move them into the widget section which has completely disappeared with all its widgets. Bizarre! Never had this problem before - can you guys advise?
I tried deactivating all the plugins and changed themes but the problem is still there. Weird. Grateful for any help on this. I have a full back up but will only resort to this if there is no quick fix.