NicoAndTheNiners Your Blissey won’t have disappeared; it will be one of the two lower ones.
There was a stat rebalance a few days ago (we’ve known it was coming for a while but weren’t sure when exactly it would hit the game).
It’s not just Blissey’s that have had their stats changed, it’s all Pokemon. However, it is much more noticeable with Blissey as they were unfairly advantaged stats wise and needed to be nerfed because PVP is coming; it just wouldn’t be workable otherwise.
Your Blissey hasn’t lost anything in terms of being powered up, it’s just the CP has dropped due to its stats changing. Blissey’s max CP is lower that what it used to be.