It appears Twitch has been hacked in some way and have issued an urgent statement asking all users to create new passwords. Here is the full text of the notice:
We are writing to let you know that there may have been unauthorized access to some Twitch user account information.
For your protection, we have expired passwords and stream keys and have disconnected accounts from Twitter and YouTube. As a result, you will be prompted to create a new password the next time you attempt to log into your Twitch account.
We also recommend that you change your password at any website where you use the same or a similar password. We will communicate directly with affected users with additional details.
I’ve just checked my account and it has been reset, I suggest you do the same as soon as possible.
Whilst most of us don’t pay to use Twitch you can upgrade and pay for Twitch Turbo, the company has not stated specifically any important information such as credit card details have been accessed.