Unable to reset password using ASP.net Membership user

I am using MembershipUser to reset the password, but membership user is always returning null
Scenario(in both cases): User have not logged on to site, accessing through url for creating password.
MemberShipUser user = Membership.GetUser(userName);    --- always returns null
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Used membershipProvider and then membership user for same password reset  - failling with exception error while resetting passowrd
WebServiceMembershipProvider memuser = (WebServiceMembershipProvider).helpermethod(applicationname)
Membershipuser user = Membership.GetUser(memuser);
user.resetPassword() - is failing with exception.
 
Please help how we can reset password