Necchi foot pedel just hum's doesn't work, why?

Only from the maker by the looks of it.

https://necchi-usa.com/search?q=walking+foot

Not available from the normal outlets. One on Amazon but not for your machine.

https://www.google.com/search?q=necchi+6018+walking+foot

..

If you don't have any movement or hear the machine attempting to sew, you may have a burnt out pedal.
You should hear a slight hum when pressing on the pedal. If you don't hear a hum, the pedal is probably dead.
These can usually be ordered from the merchant who sold you the machine.

sounds like a sew-tric plug an motor if so under the motor's electrical connection cover there is a wiring diagram moulded onto it

there could be something wrong with the motor,, make sure that
the power cord and foot pedal are securley plugged into the machine
if the machine still hums,,then take the machine back and get
another one.

Berend: Thick fabrics require a longer stitch length. If the longer stitch length doesn't work check your attachments for an Easy Feed foot (It sort of mimics a walking foot.)

Well, you are definitely getting power from the electric plug in.

Let's do some diagnostic work. Do you hear anything when you press the foot pedal? Any hum? Did you happen to leave it in bobbin winding mode?

your machine doesn't have a "sew" button for use without the pedal. So please rotate the hand wheel toward you as you press the pedal. Did the motor take over and sew?

If you know someone with a meter... have them test the pedal. (they will place probes on/in the part that plugs in which tests for shorts, etc.) If that tests ok... then we know it's something more than the pedal and it will need servicing by a Singer Tech.

try abakhan fabrics or manufacturer website or google "buy necchi 3620 foot"

www.craigslist.com

Usually pedals possess a variable rheostat controller that react with the action of your foot. If it fails to function normally, it suggests that a short circuit has occurred. Generally, that means an internal wire or contact opens up and stops all further activity as you mentioned. The best bet is to take the machine to your local sewing machine dealer for repairs.