2nd Update, months later: I fixed it. It was indeed BIOS corruption. I flashed a fresh BIOS using a USB-programmer as described here:
https://gwalkerux.com/posts/1/repairing-a-lenovo-flex-5-laptop-with-a-corrupted-bios

Update (about a week later): Sorry for taking my time. Life's quite busy right now.
I've taken the advice I got here (thank you everyone for contributing), tried a few additional things and it looks like I can confirm this to be an issue with the mainboard or BIOS-chip.
If its the mainboard, I'm SOL, since its probably not worth replacing.
I'm still 'hoping' its a corrupted BIOS, that can be reflashed. If anyone has any suggestions what I can try to make that work, you're still welcome to post here.

Original post:
My wife's Ideapad Laptop won't boot anymore.
The device is an Ideapad Flex 5 best described by it's model code 14ALC05.

I've tried to fix it for a few days already, but to no avail.

Error description:

It will power on, but screen stays black.
Not even Bios-splashscreen will show.

Initially the LED indicators on the ESC-key (Fn-Lock) and CapsLock will light up. The Fn-Lock will go dark again, while Caps-Lock LED stays on.
I can turn off the Caps-Lock LED by pressing Fn+SPACE, after which it won't turn on again.

The device will stay powered on, even for hours, no change and the LED on the power button will stay lit unless the battery runs low, in which case it starts flashing. (That's normal behavior)

Things I've tried (not neccessarily in that order):

• Disconnecting any peripherals
• Connecting external HDMI display
• pressing the Novo-Key
• holding the Novo-key
• Power drain by holding power button for more than a minute
• disconnecting the internal battery + power drain
• disconnecting the SSD

I have suspected a corrupted BIOS and tried flashing an updated BIOS by following this guide:
https://wucke13.de/posts/lenovo-bios-fun/

However, the error description doesn't seem to match exactly with my problem, so I'm not sure if I even have the right diagnosis.
I've found other pages that suggest and describe re-flashing the BIOS with an SPI-programmer. But I don't own something like that and have no prior experience, so I only want to try that as a last resort.

If anyone has got any additional ideas, what I could try or even can help me confirm that I'm on the right track, pls let me know!