Since the laptop works on the external monitor, then it would be safe to assume that it is only the L:CD display part that is the issue. Often LCD display (of laptops) are working but only that you can see the actual display. One way to verify is to shine a flashlight at an
oblique angle to the screen, semblance or part of the desktop could be
seen. If the
desktop could be seen (albeit very very dark background) then it could
either be the CCF Lamps (cold cathode florescent) or the inverter that
powers the lamps.
Often, the defective component would be the inverter. It is
relatively easy to replace the inverted. It is sometimes more difficult
and challenging cracking open the casing of the display panel of the laptop than replacing
the inverter. Additionally, inverters are readily available online. It
is also not surprising that inverters for other brands/models would
also be used in your Toshiba.This
would be a good place to start for inverter boards. This
is a nice tutorial if you are considering a DIY.
In some worst case scenario, it could be the video interface or even a loose/open flat cable from the laptop's main body towards the LCD display panel.
this be of initial help/idea. Pls post back
how things turned up or should you need additional information.
Good luck and kind regards.
Thank you for using FixYa.