Plex username policy changed earlier this year. If your account was affected, then your username was changed. See All accounts are having a unique username set .
As mentioned in that article, you can change your username in your account settings: https://app.plex.tv/desktop/#!/settings/account. The username must align with the new policy, so you may not be able to change to your old username.
You can also use your e-mail address to sign in to your plex account.
