Hello Sassy Granny,
If the machine was working before you changed the bobbin but not now, are you sure you have got the bobbin in the right way up? When you pull the bobbin thread the bobbin should turn anti-clockwise. Go to the sticky thread at the beginning of this section and watch my video about how to thread a round bobbin machine. If you still have problems after trying again, then it may be a burr on the bobbin itself.
Let us know how you get on!