Login Form Using jQuery with JSON in ASP.Net

You can refer this article

public class User
{
    public string Username { get; set; }
    public string Password { get; set; }
}

[System.Web.Services.WebMethod]
[System.Web.Script.Services.ScriptMethod]
public static string ValidateUser(User user)
{
    int userId = 0;
    string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand("Validate_User"))
        {
            cmd.CommandType = CommandType.StoredProcedure;
            cmd.Parameters.AddWithValue("@Username", user.Username);
            cmd.Parameters.AddWithValue("@Password", user.Password);
            cmd.Connection = con;
            con.Open();
            userId = Convert.ToInt32(cmd.ExecuteScalar());
            con.Close();
        }
    }
    string IsValidate = string.Empty;
    switch (userId)
    {
        case -1:
            IsValidate = "Username and/or password is incorrect.";
            break;
        case -2:
            IsValidate = "Account has not been activated.";
            break;
          
          
    }

    return IsValidate;   
}

You can take the Stored Procedure reference from this link.

Simple User Login Form example in ASP.Net