It seems like the window size is set to 0. Try holding the windows key and pressing the up arrow, that might set it back to fullscreen.
If not, you may have to edit the registry. Close the program, open your windows search and type "regedit.exe". In registry editor, do a search for "kyuppin", and in the kyuppin folder should be a vTuber Kit folder. In there will be some variables, two of which are screen width/height. Try setting those to something like 1280x720. Then run vtuber kit.
If it works, go into the gear menu and click one of the resolution buttons to make sure it sticks.
