I have an old wizard citation.. it was working great and then it stopped sewing and just makes a noise, but it still works manually.

You will need the best access you can get to the inside, so remove as many covers and doors as possible. Remove any exterior screws and that should get what comes off, off. Turn it over, if it has a bottom cover, take that off. Take off or loosen the hand wheel.

Set it on several layers of newspapers to catch the messy drips. Old grease can harden into resin, almost like rock. Some paint thinner or Varsol may help remove this kind of thing. Lacquer thinner may work too, but may also ruin the paint. It dries a lot faster.

If you happen to try to pry off lumpy bits, be very careful. You really don't want to chip a gear tooth. Finding a replacement might be next to impossible.

Hair dryer or a heat gun on the lower setting will help to soften old oil and grease also. Heat guns can blister the paint, so take care.

A soft new paint brush would work nicely to spread solvent around and into the gears and it will clean the metal nicely too. Avoid getting solvent on belts, synthetic or rubber parts. It can soften or melt rubber especially.

If you use solvent, let that drip dry. Once all the metal is clean, use oil made for sewing machines, just a drop or two, every place that metal touches metal. Use a grease made especially for sewing machines on the gears.

Once you get it freed up and running again, be sure to oil it regularly. It's the single best thing you can do for moving metal parts.

If it's an old mechanical machine, it has probably seized from old, dried oil that acts like glue on the moving parts. Get some liquid Tri-Flow Synthetic Lube and, while manually moving the handwheel, apply a couple drops every place that metal rubs metal. Do NOT oil belts, rubber, or synthetic parts. Then apply hot air from a handheld hairdryer into the internal mechanics. You may need to repeat the process several times before it moves freely.