Hello Lulu,
If your machine does not pick up the thread from the bobbin the most common causes are.
Bent or blunt needle, needle in the wrong way round, machine threaded wrong, if the needle is blunt or bent the eye of the needle will be too far away from the hook for the hook to pick up the thread, just that fraction too far away and the machine won't sew.
Ken
drivewithken@blueyonder.co.uk, July 2007