An official Google AdWords represented confirmed an issue that some advertisers were experiencing this past Monday. The issue was with some advertisers not being able to see or click on the "billing" tab within the AdWords advertiser console.
A AdWords Help thread has AdWordsPro posting this as a "known issue." The representative explained it only impacted some advertisers:
We have received a number reports from advertisers that they are currently unable to access the Billing tab. Engineering is working on the issue as a top priority, and we hope to have a resolution soon.
By the end of the day, the AdWords representative said they were 95% sure it was fixed:
And while I am only about 95% certain at this point (being the sort that likes to wait half a day after first having heard something is resolved) I think it is time to say that the missing Billing tab is fixed.
I certainly hope it is 100% fixed by the time this post goes live the following morning.
Forum discussion at AdWords Help.