Cydia's backend has been extensively affected by Amazon's EC2 outage. Towards the end of last week, Amazon's popular EC2 cloud computing service went offline and affected many heavily-trafficked services across the internet.
When EC2 went under, Cydia's store and account infrastructure went with it. Any type of account activity remained unavailable until EC2 slowly started kicking back in over the weekend.
Cydia's creator and admin, saurik, has posted an update to Cydia's homepage explaining the issues that Cydia has been experiencing for the past 84+ hours.