It looks like part of your Spine window is outside the bounds of your monitor. What OS are you using?
If you can see the Spine window title bar, try dragging it, or pressing the maximize/fullscreen buttons in the upper right.
If you can't do that, there are probably hotkeys for your OS to toggle maximizing a window.
You could close Spine and open it again, that should center the window on the screen. If still too large, you can edit or delete the json files in your settings folder, then restart Spine. That will reset your Spine settings to their defaults.
Windows: <user home folder>\Spine\settings
Mac: <user home folder>/Library/Application Support/Spine/settings
Linux: <user home folder>/.spine/settings
