So just earlier today I had my Hidden Ability Shiny Jolly Weavile with 28 speed ivs. I did a cbattle all was fine, so I decided to stop doing cbattles to ev train a yanma that I caught. So I went to gen 4 trained it and so forth, I came back to gen 1 and went to grab my weavile out of my pc and it was missing. I tried relogging to see if that fixed the problem, I went back to gen 4 to see if that fixed and nothing. I do have a picture of the pokemon within my party from yesterday, and I can get pictures of my pc if needed.
If you indeed do have the photo evidence of the Weavile, try messaging a Manager with the evidence and see if they may do anything.