did you put the bobbin in the same way you took it out ???? some machines are very sensitive. also if you used the top thread to to rewind the bobbin double check that it is re threaded properly, sometimes if it isn't between the 2 little tension wheels it can play up. if that doesn't fix it rethread the whole machine again, normaly that will fix the problem. if that doesn;t help check that you don't have a lint build up in the bobbin area.