I had this problem before...
Have you deleted any files by mistake? (I once deleted the "route" with no file extension from the route folder) If so, restore them, if not then.....
You will need to re-download BVE from Mackoy's site and save it somewhere that you will know... such as My Documents. Afterwards, run BVE and when that messagebox pops up, click on Browse and look for the directory where the BVE installer ins located (The one you just downloaded). After you have selected the installer, click OK. This will reinstall the game.