How do i fix my computer?

Awhile back i was turning on my computer just like any other day. I turned my tower on and my desktop wallpaper popped up. I knew it was that because it was a pic of my cat, but my mouse, icons, shortcuts, and task bar was no where to be seen. As long as it went to the desktop my mouse and keyboard stopped working. Nothing is working whatsoever. Just the picture of my cat. Can anyone tell me what it could be that is causing my computer to do this???