how to insert value from dropdownlist to database

hi
i have one gridview. in which i added one dropdownlist in one column  in 'onrocreated' method of gridview. i change the value from dropdownlist in any row of gridview.
this changed value i want to updated in the database.
i use dataadpter.update method, some times it gives me error like 'operation need updatble query' or sometimes won't but the data is not updated in database

can anyone help me.
thanx in advance.
-- laki