KseniBlake and anyone else who is stuck like this, I have just spent half a day panicking about what to do in your position, so here is what I did to save you the trouble.
1) Go to
https://www.mobileread.com/forums/sho...d.php?t=169645
2) Download 'KindleSelectBoot.zip' in the attatched files at the bottom of the first post.
3) Unzip and run 'MfgTool.exe
4) Select 'Kindle Main' from the dropdown menu.
5) Scan devices.
6) Press 'Start' and the kindle should go to the main menu!
Couldn't find ANYWHERE with instructions as simple as these. Really hope this helped.