I've done fashion designing & coutourier (can't spell it, but did it), & home sewing for many years. I use a sear's kenmore. The problem could be a few things - first adjust the tension (at the top pf the machine), next make sure you are using the proper thread and needle size for the fabric. Rethread the machine & make sure your bobbin isn't catching (knot etc) & pulled through correctly. Test first by hand lowering the needle through the platen/metal guide & raise it to get a loop. Pull the thread under the needle to the back. Correct or change the pressure foot if needed - you may have the wrong attachment on from the last time you worked w the machine i.e. platen or foot. Check to see if something is lose - presser foot etc. If need be, rethread your bobbin. Check your stitch length. Run the machine through a swatch first so you don't ruin the fabric. I messed up silk once, and just about had a fit. (No pun intended.)
Another thought - there should be a few platens & presser foots. Make sure you don't have the zigzag foot or platen on. If all else fails, and you have a serger, switch over to that.