|Replacing laptop parts is not easily done. |
The motherboard is very expensive.
It is probably not the hard drive. You don't need a hard drive to POST.
When connecting an external display to a laptop you may need to use certain keystrokes to enable the external display. Consult your manual.
Your GPU (video) chip could be defective. There were many nvidia GPU series that were determined to defective. Look at the link below.