Dell Insprion 1100

HI I WAS WONDERING IF ANYONE COULD HELP.
I HAVE A DELL NOTEBOOK AND HAVE A DISPLAY PROBLEM.
MY PARTNER CHANGED A DISPLAY SETTING (HE CHANGED THE DEFAULT MONITOR I THINK!!!) NOW WHEN WE TURN THE COMPUTER ON IT LOADS UP FINE, SHOWS THE DELL PAGE THEN SHOWS WINDOWS XP LOADING. HOWEVER ONCE WINDOWS IS LOADED THE MONITOR IS BLANK. THE COMPUTER IS STILL ON AND WORKING AS YOU CAN HEAR IT.