First take a deep breath. It's probably something simple.
Clean the machine thoroughly, especially the bobbin area and under the plate. Look for thread wrapped up in the bobbin area. Change the needle. Completely re-thread the machine, and check the book to be sure you are doing it correctly since you haven't used this machine a lot. Check to be sure that with the presser foot up and the needle in the up position you are able to freely pull the thread from the top and bottom. If it hangs up, there's a problem somewhere. Try just a few stitches on scrap fabric and see if the thread is still collecting on the back. Hopefully all will be working now.