Open your osu! cfg file on your directory. (It's usually osu!.X where X is your Windows Account name).
Press Ctrl+F to bring up the Find window and type in "Width" and click "Find Next". The default osu! screen resolution for most PCs is 1024x768. So Follow these instructions below.
[Windowed]
Width: 1024
Height: 768
or
[Fullscreen]
WidthFullscreen: 1024
HeightFullscreen: 768
After editing the cfg file open your osu! and see if the screen shows again. ^_^