no display on external monitor through VGA / HDMI port

I have a 14" Dell studio 14/1458. does anyone know how to resolve non functioning of the video ports - VGA, HDMI?  the usb port works fine but no video on external monitor through VGA or HDMI ports. I already checked the connecting cables and external monitor and they are ok. also ran the dell diagnostic test on the laptop and all is ok?

FYI- I got the op system reinstalled from a technician before which all was fine. now i also checked for any missing drivers from dell website but i dont see any video driver missing. please guide me if you know how to resolve it.

thanks a lot

ajay