For example:
string strUserInputtedHashedPassword = 
             FormsAuthentication.HashPasswordForStoringInConfigFile(
                                      tbPassword.Text, "sha1");
if(strUserInputtedHashedPassword == GetUsersHashedPasswordUsingUserName(tbUserName.Text))
{
   // sign-in successfull
}
else
{
   // sign-in failed
}

//The following live sample will create a Hash of your inputted string value, the source code is //also below. 
 
protected TextBox tbPassword;
  protected Literal liHashedPassword;

  
  private void Page_Load(object sender, System.EventArgs e)
  {
   // Put user code to initialize the page here
  }
  public void btnHash_Click(object sender, EventArgs e)
  {
   if(tbPassword.Text.Length > 0)
   {
    string strHashedPassword = FormsAuthentication.HashPasswordForStoringInConfigFile(tbPassword.Text, "sha1");
    liHashedPassword.Text = "Hashed Password is: " + strHashedPassword;    
   }
  }
