There are many, many Google AdSense publishers who are complaining about not being able to login to the AdSense console. There are threads at Google AdSense Help and WebmasterWorld with many folks complaining.
The issue first started yesterday at about 5pm (EST). Shortly after, people began noticing that if you login at https://www.google.com/adsense/login3 it does allow you in. The default login area seems to not work for many. I personally tried it and it works.
You should also try to delete the cookies related to the AdSense login area. If that doesn't work, try logging into https://www.google.com/adsense/login3.
Google has not confirmed this issue, nor have they replied to questions and concerns about it.