Hi, make sure you are using the correct needle for your sewing machine. If you have verified that you are using the correct needle then your timing might be off. The needle only has to go in far enough so the hook can catch the thread. You can visually inspect that by removing the needle plate. If the hook does not revolve then a gear may be broken but you would have to check and see what happens between the hook and the needle.
Best regards
Ben M
www.sewingmachineryrepair.com
Benjamin
January 2010