You have two issues...one you are not coding defensively so are prone to these errors. There is no guarantee that your session variables exist so you have to check first. Secondly it looks like you might be storing ints as strings, which you shouldn't. To store an int use
Session["session"] = myIntVar;
to read it back;
int x = 0;
if (Session["session"] != null)
{
x = (int)Session["session"];
}
That'll stop the error, but now you code needs to know what to do when there is no session variable, it has to do something sensible like show a message, redirect to a login page, whatever.
Your next issue is why the session is empty. It could be that godaddy are reducing your session timeouts quite low, it could be that sessions aren't working at all. We don't know.
Lastly, good luck with your weight issues but please stay away from snake-oil products from the internet :)