It's entirely possible that your account was hacked and somebody changed the email address. But if
that happened you should have received an email indicating that your address was changed, and telling you what the old address was.
The other possibilities are that you typed your address differently when you tried to log in, or (never discount the possibility) there might happen to be a bug that's affecting you right now.
The first thing thing to do is check your email for messages sent from GOG. If there's nothing out of order then try log in again, being careful with your email address, and trying a "forgot my password" with that address if that doesn't work. If GOG still doesn't accept your address and didn't send you a password reset email then contact support
and hopefully they can help you.
When you get access to your account again, it would be a good idea to enable two-step login
Good luck. Let us know if you get things sorted out.