Unplug the system, disconnect the battery and hold the power button for 30 sec.  Reconnect the battery, plug in the system and boot -- see if the discrete GPU appears.  If it does not, it has failed;  you will need to replace the system board.  
I am facing this problem that started suddenly more than a day ago. Already did the BIOS downgrade, already returned the system to factory default, already disabled the BOOT SECURE in the bios, I tried to activate the hidden options in " Device Manager" but there shows nothing about my video card, already updated my bios to the latest version, but without success. It is only recognized the onboard Intel UHD GPU and nothing appears related to the RTX 2060 Not even third-party programs recognize my video card, the same happens when I access the BIOS. Speccy Informations   Please note that my windows are updated along with the bios to the latest version. I am desperate for a solution, also strange that the HDMI input is not recognized as a second monitor, so I can not use it either. Please help me, I'm desperate.  
DELL G5 5590 - Doesn't recognize the graphics card (RTX 2060)