My neighbor recently had a problem with her sewing machine and I offered to check it for her. It turned out she had two different sized bobbins, both plastic and almost identical, and she was using the wrong one. With the right bobbin her machine sewed beautifully. I'm not suggesting that's your problem, but before taking the machine in for a checkup I would check all the more obvious things.
When you say it won't sew, can you be more specific? Does the needle go up and down? Does it pull up the bobbin thread?