Oil turns gummy or even into a varnish like substance when it sits long enough. Take the top and bottom covers off the machine and oil everything where two pieces of metal move against each other. Use Tri Flo synthetic oil or sewing machine oil. Wiggle the machine every time you pass by it. Eventually the oil will soften up the dried oil and your machine will start turning. Be patient. It took a long time for the old oil to dry up. It will take a while to free up again.
Rodney