monitor glitching

For a couple of weeks over christmas break i hooked up a second monitor  to my laptop (inspiron 9100) at home just to give myself a bigger desktop.. etc.. then i came back to school and dont have the second monitor anymore its just my laptop, and whenever i open up a menu the menu glitches.  this didnt happen before i had the other monitor hooked up.  its not the whole screen that glitches and it only happens when i open a menu, and its only the menu that glitches? what to do about this?