Amanda, when you run the machine without the needle going up and down, the belt is slipping and will break.
It seems that you have cotton or something in the bobbin mechanism.
Ensure the needle is up, Remove the bobbin and its holder then release/turn the two clips and remove the part that holds the bobbin and its holder, by this time you should have some idea of the problem.
There are other types, the above may not apply to your machine but the problem will nearly always be in the bobbin area.
jim, December 2004
