Forgot my ubuntu password..

So many seraches and finally got an easy solution.
1)Start your Ubuntu in recovery mode

2)Select Drop to root shell prompt from the window that appears. Sometimes that window appears for a very small duration. So keep pressing DOWN-ARROW key after you start in recovery mode.

3)A prompt will appear.
# passwd your_username
# passwd new_password
# sync
# reboot -f
And you are done!!