Session problem

Hi guys,
I am having certain problems in session. Whenever I make a query to the users table
like 'SELECT * FROM {users} WHERE uid = %d',$uid something like this then the session is destroyed and the user is logged out. Why is it so??
Actually what I am trying to implement is like this
I am developing an ecommerce site. So when the users checkout their cart then I redirect them to an email form where they can email their queries and request to the owners of those products. From the form I get the products array.By using node_load I load the node corresponding to each product. Now to get the mail and name of the owner I am running the query given above and that is where I am getting the problems.
Seeking for help