I think I know what your bird has. My Muscovy had trauma as a duckling to the head and it neurologically messed him up. Same with another chick I had. It causes them to suddenly go into shock or have sudden seizures. Sadly both of mine passed away.
They can be as normal as can be, walking around and just lock up. I'm thinking it could be a stroke, also. And that when they get better it's just like physical therapy for a person. My Muscovy didn't start having these seizure till he was able to fly. One day when in training ( yes, I train my ducks to fly ) he's soared and finally learned to fly! He took off over the house and around and had a perfect landing into the pond and came up to me for his treat! Next week we tried again, I walked up onto our deer blind and tossed him into the air. He soared around, took up into the clouds as happy as ever. Then it happened. He locked up in mid air, super high up. He started a nose dive and crashed. All the other muscovies came over to see him. ( you know how when a bird dies they have the " spaz " thing? ) he started twisting his neck and flapping when I looked over, I got inside the coop and started crying. I thought I had just murdered him, " Stupid Training," why didn't I just clip his wings...." Then I looked up and he was sitting next to me waiting for his treat! I was astonished! I was like " wha-what! I'm so seeing things..." but no he just had a seizure and locked up. He eventually died after 17 seizures 3 days ago.
Dry rice (about a 1 1/2 cups) in a cloth sack (or old sock) works well heated in the microwave for 1-2 mins.
