Just taking a brief look at how your code is set up and I think I can point you in the right direction. I personally didn't feel like downloading something to view the sql database or uploading it so I can't look at that part but generally when an sql query fails it's due to an invalid field in your query or the query itself being improperly formatted.
So if I had to take a guess I'd say either one of those fields isn't set up in your database (or is the wrong type) or your query string has an error in it. I don't think breaking it up like that was really the best way to go about it personally. If your database has all of the correct fields and types try:
$qry = "INSERT INTO user (firstname, lastname, username, password, email) VALUES ('".$_POST["fn"]."','".$_POST["ln"]."','".$_POST["un"]."','".$_POST["pas"]."','".$_POST["em"]."')";