Did it ever work? I just logged in with NO issue. How and on what device are you trying to log in? Have you tried different methods/devices?
I have had service for two years and never had a problem logging in to access My Account. I tried different browsers, my iPhone, iPad and every which way possible. Just tried again today, and still no luck.
You may need to call and have a tech (if you can get to one) ensure your credentials are linked to your account properly. This has been a solution in the past after changes were made to the system leaving credentials unlinked.
Comcast is currently working on the problem since it's so widespread. Keep an eye on the following topic for more updates: