Hearthpwn/Curse/DiabloFans/whatever merged into one. If you didn't log in to your specific ...-account within the given time it means it became inaccessible.
And that is because you didn't merge it with a Twitch account back then. See an "Hearthpwn" email example down below:
We’re emailing because we noticed you haven’t merged your HearthPwn account with a Twitch account. Starting July 30th, you will no longer be able to log in to unmerged Curse accounts. This means unmerged accounts and content created on those accounts will be inaccessible.
If you’d like to retain access to your account and content, please visit *Expired link* and log in before July 30th. When logging in, you’ll be presented with the option to merge your Curse account with a new or existing Twitch account.
After merging, you’ll be able to continue accessing your HearthPwn account by logging in to the site with Twitch!