Open the faceplate on the side, poke the swing arm and see if it moves freely. Open the top and inspect parts, give all the oil points a drop of oil, equally so behind the faceplate. The freearm pops off too, and you might as well oil everything if it's been a while.
Turn the stitch selector lever on top to straight-zigzag, gently turn the stitch width know and needle position knob and test sew a bit. Move the stitch length selector lever and try a few different stitch patterns, and see if the cam stack turns. I'm guessing it's stuck some where and parts needs a bit of tlc to shift and turn freely again. Test sew a bit again, gently poke moving parts under the top and behind the face plate, and yet another round of oil.