Hello, my side centos6.8 system boot indicates that "the or type Control - D to continue", need to enter a password on my side, I enter the root password is always wrong (don't know if someone change), the message is I "grub" into the interface, edit the input command "single" or "1", press the B key to restart cannot into single user mode to change the password, restart or tips "or type Control - D to continue", I want to ask this is what causes, because the system has data needs to be saved, need to back up your data into the system, can have what good way to teach, hard disk, and array are normal, also can enter the BIOS, trouble to give directions, thank you