screenshot not working

The screenshot on my Macbook Pro has not been working for a couple weeks now. Both the full screenshot (command+shift+3) and the partial (command+shift+4) are producing pictures of colorful vertical lines rather than what's actually on the screen. The .png file that is created and automatically placed on my desktop is the right size for the shot I'm "taking," but the image isn't there! Please help; I need my screenshot back!