Configuration :: Website Can't Display Page?

I was trying to get a webservice to display the content of a table, but keep getting HTTP 500 message. Here is my code: (The code builds OK, the problem comes when I Invoke the webservice)

[WebMethod]
public
DataSet ServiceRequestStatus()
int? customerid =
null;
string username
= User.Identity.Name;
GOBdataContext dc =
new
GOBdataContext();
dc.GetCustomerID(username,ref customerid); //Stored Procedure
if(customerid == null) {
throw
new
Exception("Error: Invalid username!");
}
var servicerequest = from cust
in dc.ServiceRequestMasters
where cust.CustomerID == customerid
select cust;
return (DataSet)servicerequest;
}