How to reset password in membership asp.net

Hi,

I am trying to reset the password but not able to do it.

Placing in page_load

MembershipUser usr = Membership.GetUser(username);
string resetPwd = usr.ResetPassword();
usr.ChangePassword(resetPwd, newPassword);

changing name of the username  to the username which I want to change the password.

But it's not working, error says" username does not exist even it's there.

Can anyone help me in this please.