The hand wheel spins but the needle doesn't move on my singer 20U73

1. Disengage the bobbin winder.That is done by moving the arm away from the center wheel. 2. Tighten the hand wheel. When you tighten you should have resistance like a jar has. If this doesn't then continue. 3. Loosen the small screw attached to the hand wheel center lug. Now, move the hand wheel into a position where it will tighten. 4. Go ahead and tighten the small screw. Now, it should move.

is it the original bobbin? try a different bobbin, sometimes they won't work if slightly dent or chip! I know it sounds weird bt it's the truth about bobbins

is the needle the right one for the fabric being sewed?
is the needle turned the right way in the needle holder and pushed
all the way up in the needle holder?
if the above is totally correct, the I suggest taking the machine
to some other repair person, cause if the repair technican can't
time the machine they should have told you they couldn't work on the
machine .

Robin. Take off the hand wheel. Look for the funny looking washer with ears. The center fingers should be angled outward when you fit it into place (counter intuitive, I know) Fit everything back together and try it again.

go to http://www.singerco.com/accessories/instruction-manuals and type in your model number, then click "search" to locate the manual for this machine.

Sounds like the clutch is disengaged on your flywheel. Hold the outside of it and turn the inside bit away from you to reengage it.

I had the same thing happen on a 14j334 serger, and the only place I could find one was on www.sewingpartsonline.com, but I don't see your machine listed. Try calling them or Singer at 1-800-4singer and they may be able to advise you as to what size it is.

try this link http://wiki.techshoprdu.com/index.php?title=Singer_20u

for user manual, service manual and parts diagrams.

Firstly change the needle.
However, if this doesn't fix it, then t may be that the timing is slightly out
there is service, parts and instruction manuals available at this link
http://wiki.techshoprdu.com/index.php?title=Singer_20u

I trust that this helps you resolve the issue with your 20U

Make sure the lock is tight. If it is even slightly loose, the wheel will spin without moving the needle.