There are several long-necked dinosaurs. But you might refer to the most common one (from popular fiction as Jurassic Park), named Brachiosaurus.
Apatosaurus, formally called brontosaurus, is likely the dinosaur you are looking for.
If it has four legs and eats plants, it is a sauropod. If it has two legs, it may be an ornithomimid, Coelophysis, or Therizinosaur.
brontosaurus
Apatasaurus, Brachiasaurus,Diploducus
Sauropods
Amargasaurus.
Argentinosaurus was the biggest hervivorous dinosaur that has a long neck.
A long neck dinosaur that eats plants
grass and leaves
Diplodocus.
Plesiosaur
This depends on which long-necked dinosaur you are talking about. Heights varied between species.
Janenschia is a dinosaur. It was a huge dinosaur with a long neck and tail.
mamenchasaurus
it has a long neck
There were a number of long-necked dinosaurs. One was the brontosaurus.
One particular Dinosaur called Hadrosaurs (Duck-billed dinosaur)