I planted a large leaf hosta this spring and it was blooming. The flower head was tall, huge and just starting to open. Today it is completely gone!! What could have eaten it?

Rabbit, deer, groundhog.

Gerry, I fortunately don't have any of the three critters you mentioned, but I'm curious... do groundhogs eat flowers?

My guess would be deer, only because of the height of the bite mark, but do deer ever eat hosta blooms while leaving the leaves alone?

Yes groundhogs eat Hosta flowers. They love them. It could be any of the above suggested. What happens sometime is they start to eat and get scared away before they get to browse.

When you see a plant eaten they will be back because they know there is a lot of good stuff left. They have just started shopping they will be back to pick up the groceries.

Cinta, that's good to know!