error: "Object reference not set to an instance of an object." - asp.net

Question

I am new to C# and I am having a hard time solving an error. When I log in, I get the error:
Object reference not set to an instance of an object.".
I know this means the table does not exist, but how do I correct it? My Database contains 4 tables and the table I am referring to is the third table in the database(I don't know if this will help). Please tell me if you need more information. I think the problem is with the loop somewhere, because I get no syntax errors.
My login code in my web user control(just for background information)
<%# Control Language="C#" AutoEventWireup="true" CodeFile="WebUserControl.ascx.cs" Inherits="WebUserControl" %>
<% if (Session["Patient"] == "Logged In")
{ %>
<asp:Button ID="Logout" runat="server" Text="Logout" onclick="Logout_Click" /> <br /><br />
<a>Continue to:</a> <asp:LinkButton ID="homepage" runat="server" PostBackUrl="~/home.aspx">Home page</asp:LinkButton>|
<asp:LinkButton ID="book_appointment" runat="server" PostBackUrl="~/appointments.aspx">Book an Appointment</asp:LinkButton>
<asp:HyperLink ID="HyperLink1" runat="server" Visible="false">HyperLink</asp:HyperLink>
<% }
else
{ %>
<br />
<a>Username:
<asp:TextBox ID="username" runat="server"></asp:TextBox> <br /><br />
Password:
<asp:TextBox ID="password" runat="server" TextMode="Password"></asp:TextBox> </a><br /><br />
<asp:Button ID="Login" runat="server" Text="Login" onclick="Login_Click" />
<asp:LinkButton ID="linkRegister" runat="server" PostBackUrl="~/register.aspx">Register</asp:LinkButton><br /><br />
<a><asp:Literal ID="literal1" runat="server"></asp:Literal></a><br /><br />
<% } %>
This is the code behind on my login control:
protected void Login_Click(object sender, EventArgs e)
{
cQuery record = new cQuery(); //declare instance
record.Sqlstring = "SELECT username, password FROM dbo.patient_details"; //what to get from database -- sql query
DataSet data = record.SelectStatement(); //put info into dataset
DataTable table = data.Tables[0];
foreach (DataRow row in table.Rows)
{
if (row[0].ToString() == username.Text)
{
if (row[1].ToString() == password.Text)
{
Session["Patient"] = row[0];
Response.Redirect("myaccount.aspx");
return;
}
}
} }

Answer 1

I know this means the table does not exist
No, it doesn't. if the table did not exist yo would get a SQL error.
What it means (assuming it's occurring in the method you posted, which you haven't confirmed) is that one of the variables below is null:
row[0]
row[1]
username
password
Which I doubt unless you have a record with a null password, or the username and password controls are created dynamically. Look at the stack trace of the exception and you should be able to pinpoint where the error occurs.