The OS has been corrupted. Backup your files (which you should be doing anyway) and reinstall. The W10 installer is free, activation will work if you have activated it before.
I am using Windows 10 but a message is showing on my screen right bottom corner.
I am following this guide: https://www.hows.tech/2017/03/how-to-remove-activate-windows-watermark-on-windows-10.html
I am not able to open regedit, not sure if regedit is installed in my computer.
Can you please help me here.