Feed dog on Necchi 681c has drop,how do I fix it?

There is a button with two "prongs" or wings. Rotate the prongs clockwise about a quarter turn, to face the square mark (Q in the diagram) then press it down. It will stay down. To raise the feed dogs, turn the prongs anticlockwise (to S in eh diagram) and it will pop up
https://www.manualslib.com/manual/1205574/Necchi-Automatic-Supernova-Ultra-Mark-2.html?page=7#manual

If you raise the presser foot, set the stitch length to a 3 or higher, then slowly press the foot control, can you see the feed dogs rise, move backward, then drop and come forward again? If they are not moving, the feed dogs have seized. If they are moving but do not rise above the needle plate, then you have them disengaged.

If they are seized, resist the temptation to run the machine anymore until you can get it serviced. It could damage the motor causing more repair cost.

If they are disengaged (dropped), then you need to find the control lever or dial that engages them.

I suspect the knob on the lower right hand side is the feed dog knob. The "knit" symbol is for the feed dog. One setting drops the feed dogs so you can move the fabric yourself, ie free motion quilting. The other setting raises the feed dogs so the machine will feed the fabric for you.

to find a manual, Search the internet:
Brand + Model + the word "Manual"
...

https://www.google.nl/?gws_rd=ssl#q=necchi+681c+parts

https://www.google.nl/?gws_rd=ssl#q=necchi+681c+instruction+manual

try these sites
sewvacdoctor.com
a1sewingmachines.com
sewingpartsonline.com

If you do not have a button or lever on the outside you may have a button or lever under the slide plate, if you have neither then you will have a raised plate in your attachments

Tighten the screw under the plate near the feed dog

the presser foot or what ever foot your sewing with does not feed the
fabric,, the feed dogs in the needle plate are what moves the fabric
to be sewed, the feet only hold the fabric against feed dog so the
machine can use the needle to sew the fabric.
if the fabric is not moving then the feed dogs have to be checked
can you see the feed dogs in the needle plate area are the up or
down?
do you have a manual for the machine?
a manual help to know where the feed dog lever is, I'm sending
you a link for a manual for the machine, so you can look for the feed
dog lever, if the feed dogs are up then there is something wrong
with the feed dogs & they need to be checked.
but if the feed dogs are down then look in the manual & see where
the feed dog lever is & put it in the up position & see if that brings
the feed dogs up to the sewing position, if the feed dogs do not
come up then the feed dogs need to be checked.

http://www.sewingpartsonline.com/instruction-manual-necchi-6015.aspx

What is your stitch length set to? Is it possibly set to a low number? should be 2 or 3 for a normal stitch.

There is a website for the USA Necchi importer, http://allynint.com/index.html, Contact Information
Maybe they can help you obtain the manual for this machine.

The Necchi machine has a lever in the bobbin area that lets you move the feed dogs up and down. If you accidentally moved that, the feed dogs are in the down position and won't feed the fabric forward. Make s ure you can feel the feed dogs slightly above the plate. If they aren't move the lever until they do. :)