The INSERT statement conflicted with the FOREIGN KEY??

Everyone is noob when learning something new so don't feel bad because I was once a noob too :) That's why you need to understand how stuff works and learn from it one step at a time

Before referencing a Session value make sure that you check for null first Your Session may/will return string but you should do a null check on the Session object itself like so:

if(Session["KeyName"] != null){

     //your code here to access the Session value

}

The error message says you are trying to insert data that has a foreign key value that doesn't exist in the parent table. For example, you might have Users with Id values of 1,2,3,4 and 5, but you are trying to add a Thread with an Users of 6 (or, more likely, 0). You should use a debugger to double check the foreign key value.

Adding to that, I will not recommend you to append the TextBox values directly into your SQL statements because that would leads you to SQL Injection , use Parameterized Query instead for security and more cleaner codes.