Click on the Launcher once and then click on the „Repair" Button.
This.
If it does not help try the following:
Open your "My Documents" folder
Find your Mount and Blade Warband folder
Opent the rgl config.txt file inside it.
Scroll down a tiny bit and you will find this:
Altering these values manually should result in the Resolution to be changed ingame.
I have never done that myself, so do it at your own risk.
Also try to make sure to change it to some real resolution values, nothing like 1200x50 or something.