thanks a lot for your answers.
fortunately, I got scared for nothing - I pressed the reset button with a very narrow lead, it didn't do anything and then i closed it and now it works. This after staying stucked for more than 4 hours trying to reset it or at least to close it. I have no idea why it worked now, but I'm glad I have my e-reader back again
After I managed to close it, my laptop recognizes its contents just like before.