forgot password for another one user account created in my system, how to solve it?

I have two user accounts and forgot one user's password. I have tried to reset it from the boot menu. After selecting recovery mode I have selected Drop to root shell prompt option but then it asked me to "give the root password for maintenance(or type control-D to continue)" don't know the password and even typing control-D doesn't work

What should I do now??

Please give me a solution