That is really a mystery.  If you are 100% sure VGA and HDMI cable to monitor are all working, then the only tests I think left to do is try this: remove both ram, does power button flash amber or pc beep then? install one ram back, try remove cmos battery for 30 min, drain system power by pressing power button and unplug power cord finally clear cmos settings by reset cmos jumper (#14 on board layout) on motherboard.
Yesterday, my Dell Vostro 3668 ran normally but now, the power light is on but the hard drive led is not blinking and the display shows nothing, how can I fix it at home while coronavirus? Service Tag:  my kid tear the express code, I'm sorry I posted with my phone 
Power Light on but nothing happen...