I just bought a singer 20u73 sewing machine and instantly lost the screw that hold the needle in the shaft! Can I get a machine screw at a hardware store to replace this? what size? HELP! What do i do?

Needle Clamp Screw.
Do you recognize it from this eBay selection?
http://www.ebay.com/dsc/i.html?_from=R40'R40'R40&_sacat=0&LH_TitleDesc=1&_nkw=needle+clamp+screw+Brother+831&_trksid=p2045573.m570.l1313.TR0.TRC0.H0.Xneedle+clamp+screw+Brother+831&_osacat=0&_odkw=needle+clamp+screw+831

This is one other possibility:
Flip Up Two Needle Clamp FL701 1 4 For SINGER ADLER Other Sewing Machines...

If you lost the screw down into the machine, you need to get it out or terrible damage will occur.

sounds like it's out of time. you may want to take it to a repair shop and have them take a look at it.

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.

Call Singer at 1-800-4singer and ask them.

try this link http://wiki.techshoprdu.com/index.php?title=Singer_20u
for user manual, service manual and parts digagrams for this model

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

for user manual, service manual and parts diagrams.

some needle clamp screws have a indention where a screwdriver can be put in to turn the screw,,
check to see if your has the indention & if not you need a new needle clamp screw

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

OK, this is a general timing procedure. Loosen the screws on the main shaft gear that goes to the hook drive gear and separate the gears. Next put the needle in the left position and bring the needle all the way down by turning the handwheel. As the needle comes up make the point of the hook enter the scarf of the needle (just above the needle eye)and hold it there while you put the gears back togeather and tighten the screws. This should put it close to being in time. To fine tune it you can loosen the needle bar screw (half way up the bar that holds the needle) and adjust theneedle so the hooks point is just above the needle eye. You may have to do this a few times in order to stop it from skipping stitches. Good luck. Rick