I guess youre talking about wrpedit... I got the same thing and hink it is if you hold your mouse pointer near the edge of the window and click on it when in prewiew.
For me the camera works again the next time I start wrpedit so try save your work close it and reopen.. To prevent this problem I always browse my work in full screen mode in wrpedit.
Hope this helps.
(If not look what your height coordinate is for the camera and try st the position manually..)