XPS 13 9343 eight beeps, doesn't boot to external monitor

Hi Community, 

I have an XPS 13 laptop, and recently it doesn't boot anymore, instead giving off 8 beeps, which I learnt indicates a display failure. 

I'm now trying to boot it with an external monitor. Upon booting, the external monitor shows the DELL sign as usual, but the laptop screen remains dark, giving off 8 beeps and switching off after a while. I tried pressing F8 or Fn+F8 but that doesn't help.

I tried pressing D when powering on, and then the laptop screen goes through a sequence of colours (with the external monitor remaining dark, no DELL sign). 

Does anyone know how I could get the laptop to boot? Thanks!