Thanx Bekham for your reply. I got the solution and reset the password through GRUB menu. Here is the solution.
To reset the root user password in the vCenter
1. Boot the virtual appliance and navigate to the console for the virtual machine in the vSphere Client.
2. Click in the console and press any key to display the GRUB menu.
Note: The GRUB prompt remains on screen for 7 seconds before it starts the boot sequence.
3. On the GRUB menu, select “FailSAFE”.
4. Type e to edit the line. A list of items in the GRUB configuration file appears.
5. Select the line that starts with kernel and type e to edit the line.
6. At the end of the line, press the spacebar and type init=/bin/sh.
7. Press Enter to exit edit mode.
8. On the GRUB screen, type b to boot into single-user mode.
The virtual appliance boots in single-user mode.
9. To change the root user password, type passwd root and follow the on-screen prompts.
10. To restart the virtual appliance, type reboot and press Enter.
When the virtual appliance restarts, you can log in using the new password.
Glad that you got the solution.