Reboot the vCenter Server appliance using the vSphere Client.
When the GRUB bootloader appears, press the spacebar to disable autoboot.
Note: If the time between when you power on the virtual machine and when it exits the BIOS or EFI and launches the guest operating system is too short, you can adjust the delay. For more information, see Delay the Boot Sequence in the VMware vSphere 5.5 Documentation guide.
Type p to access the appliance boot options.
Enter the GRUB password.
Note:
If the vCenter Server 5.5 appliance was deployed without editing the root password in the Virtual Appliance Management Interface (VAMI), the default GRUB password is vmware.
If the vCenter Server 5.5 appliance root password was reset using the VAMI, then the GRUB password is the password last set in the VAMI for the root account.
Use the arrow keys to highlight VMware vCenter Server Appliance and type e to edit the boot commands.
Scroll to the second line displaying the kernel boot parameters.
Type e to edit the boot command.
Append init=/bin/bash to the kernel boot options.
Press Enter. The GRUB menu reappears.
Type b to start the boot process. The system boots to a shell.
Reset the root password by running the passwd root command.
Restart the appliance by running the reboot command.