Usually, when there's a problem on the bottom of the fabric, it means there's a problem with the top thread. Somehow, your upper tension isn't doing it's job. Sometimes, it looks like you've threaded between the tension disks, but the thread can get off to the side somehow. Visually check to see if it's in between the tension disks, if you can. Another thought is to fold a piece of fabric and run in on both sides of the tension disks to remove any lint that may be stuck in there. I hope you're able to get your machine going.