It sounds to me like your upper thread tension is off. To verify, sew a bit on a test piece of fabric with different color thread in the top and bobbin. I am also thinking you might have something jammed in the tension discs. with the foot up, take a piece of strong thread and 'floss'. When you rethread your machine, your presser foot needs to be UP for the threading process, until you get to the needle. Give the thread a small tug to seat it in the discs, put the presser foot down. at this point you should not be able to draw the thread thru the machine. Then thread your needle.