I get it sometimes also. Physically turn phone to landscape position and back to portrait. It should go back to normal. For some reason the wallpaper sometimes doesn't fill the whole homescreen. Message Nova developers. Some sort of bug.
Thank you for the reply, however I've turned the phone off and back on and even tried a different wallpaper and that black bar is still there, I even tried using a stock wallpaper and it doesn't disappear.
Have you tried a soft rest? You know, hold the power key for a few seconds until the phone options pop up shows, select restart. I cleared mine once like that. I also use a photo from my gallery for my home page wallpaper. All my photos are on my SD card so maybe that has something to do with it. Mine doesn't stay on permanently and does it once in awhile but it does look exactly like yours when it does. I'm using Nova Prime launcher and haven't found any settings that may cause this problem.
If you're using Nova Launcher, they just released an update. Did you download it? If so, try deleting all updates (app manager), wait a few minutes, then download the current updates. I use Nova and have never had this problem.