Is the thread aerial up? Is the presser foot down? (Yes, I've done both of these!)
1. Check your tension settings haven't changed. 2. Is the thread is in the tension discs? 3. Rethread your machine (presser foot up, in the order specified by your manual).
If the fabric is rippling or gathering, adjust the differential feed. If you're experiencing skipped stitches, try changing the needles.
Clean out any lint of fluff that has built up (where does it all come from???). Floss the tension discs and clean the feed dogs while you're at it.
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