I was carving my pumpkin and I messed up and knocked out a part I did not want toknock out is there some way to glue the piece back on?

Throw the pumpkin in the trash and buy a new one

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Break a couple wooden toothpicks in half and stick one half into the pumpkin and the other into the part you want to put back. 

Most of the garden shops have Apex Pumpkin Glue, an excellent product that does a great job and it's reasonably priced. Any clerk can direct you to the display so just ask one.

Use the blood of your victims, mwahahahaha!

Tooth picks will work 

Stab a toothpick or two into the nearby material and stab the cutout bit onto the toothpicks.

Use toothpicks on the inside / where they will not be seen. -Or duct tape, and make the repair (design) look intentional.

sure -- epoxy or superglue might work

Wire it back in or use cocktail sticks. Or nylon fishing line. 

Glue won't work because the flesh is too wet.

Try pushing tooth picks through the edges of the hole & into the broken piece to hold it in place.

You may have to use a pair of pliers to push the tooth picks through the tough skin of the pumpkin so be very careful.

Needle nose pliers will work best of you can borrow a pair of them.