Free manual here:
http://www.searspartsdirect.com/part...mManualTabPage
Shows oiling diagram. Needle threaded from front to back? Is the needle set correctly? Did you remove the feed dog plate and clean under it? If you've had any needles break it may need a tune-up/adjustment. There could be a thread stuck in the tension area... Did you try loosening the bobbin tension? Did you thread the machine with the presser foot up? Using correct needle size for fabric weight? Bobbin area clean and bobbin placed correctly (unwinds clockwise)? Are you using a quality brand needle? Some machines are picky and cheaper needles can cause problems..
If I think of anything else I'll be back!