Problem with Tea Touch Lux 3 e-reader

Maybe it's because the e-reader has a problem, or the computer, but here it leans more towards the e-reader.
Try it on another PC to compare the reaction.

I tried on another computer and I have the same problem.
And I can't find how to reset this e-reader.

Okay, so it's the e-reader.
But you haven't tested what I asked you to.

+ Have you charged it for at least 2 hours as recommended by the manufacturer before conducting your various tests?

And to avoid being stuck, try this (with your description we don’t know if it doesn’t start at all or if it doesn’t start all the way):
Turn on the device by pressing the power button. During startup, press the buttons simultaneously and hold them down until the message Firmware update... appears on the screen.

Oops, my response didn't go through.
I was adding that to be sure, find another SD card (see the manual for the type) and test it in the tablet.
If the issue is the same, check if the tablet is still under warranty (2 years) and use that warranty; if the problem does not occur, you only have to change the SD card.

Be careful, sometimes a bad contact comes from slight oxidation or dirt on the card contacts.
Always cautiously, remove the card, and with a cotton swab and some rubbing alcohol, gently clean the contacts of the card several times, let it dry before testing.