forgot system root password ? don’t worry :)  
Reset root password on RHEL as ber below steps (screenshots)
On the grub menu select the your kernel as in the screenshot and press “e” ( without quotes) to edit the parameters. 
Select the line with “kernel” or “kernel /vmlinuz-2.x.x.x-xx ro root=/dev/VolGroup00/LogVol00 rhgb qui->”  press “e” to edit the line as as show in [...]