Website datamissing issue

I am using Session. this was not the session issue..All the data are missing for sometime. all the dropdownlists are show as empty lists in all the pages..in all the user accounts.

But how can we found if it is a virus problem. All the users are logged out at the same time, and the log in after same time if we refresh the page..please give me a solution

I found ,why the issue is happening. Some times if there is multiple users in the website, the data from one module is replace with the other module data..so i think the issue is related to the DAL structure..

My Function structure is like this
public static DataTable ABC(string UserName, out string Error)
{
Error = string.Empty;
objDataTable.Clear();
objDataTable = new DataTable();
DataCommand.CommandText = "SELECT restId FROM tblRestaur AS rb JOIN tblOwner AS o ON rb.ownerId = o.ownerId WHERE o.ownerUserName = @UserName";
DataCommand.Parameters.AddWithValue("@UserName", UserName);
DataCommand.CommandType = CommandType.Text;
try
{
if (DataConnection.State != ConnectionState.Open)
DataConnection.Open();
DataAdapter.Fill(objDataTable);
}
catch (Exception ex)
{
Error = ex.Message;
}
finally
{
DataCommand.Parameters.Clear();
DataConnection.Close();
}
return objDataTable;
}

What will be my mistake..plz help me