Hi ondra,
You can try using another working computer to download the ISO file to create the installation media and refer to the steps provided in the link below to help recover your computer. If you do not have another computer, then you can use someone else (e.g. a family member, friend, etc.). Or you can contact the relevant computer repair person to assist you.
Reinstall Windows - Microsoft Support
Disclaimer: At this point, I recommend that we try to perform a clean install to get your computer back into a working condition. Please ensure that you backup any important data, including Documents, Pictures, Videos, and more.
Feel free to post back if you need further assistance.
Sincerely
Mavis - MSFT | Microsoft Community Support Specialist