I used the search, but didn't find anything so here the problem is:
At least in the campaign IdF ( German, I don't know how its called in english), there is a Bug that happens everytime when you load a savegame during a fight using ctrl+o.
Everytime you do this, after loading, the click of the mouse doesnt work any more so you cant move units etc. You can still access the Menu, but don't do anything on the actual map... the only way to fix this as far as I know is to quit Wesnoth and restart it...then it works great again.
About the Campaign, that is the one where the Undead attack from the east and you are human..i don't know whether this helps a lot.
Just wanted to report it.