1. power the computer and into grub-select-face.
2. select the OS and input e
Fedora (...)
Ubunto(...)
Windows
3. select line which begin with "kernel" and input character e.
root (hd0,6)
kernel /vmlinux-*** ro root=UUID=***
initrd /initrd-*.img
4. you will get a run command, then input whitespace and single, like as
*** rhgb quite single
5. input enter and waiting for shell enviroments, then input passwd, reset it.