Google has posted a message on its Checkout Merchant Forum explaining why many merchants have been unable to access their own accounts:
A recent server update has temporarily disabled the Google Checkout merchant frontend servers. As a result, logging in to your Checkout merchant account now returns a 404 file not found error. Please note that this issue only affects the Google Checkout web interface, all other services are still functioning correctly. This issue does not affect your your user’s experience with Google Checkout or your ability to accept orders and process them via our APIs.
Since yesterday, sellers have been complaining that they were unable to access the admin area for Checkout, which was returning a 404 error. Buyers were still able to place orders and pay using Google Checkout, but merchants could not check order details nor see if the payment had been verified. Most frustrating seems to have been the lack of a quick Google announcement of the reason for the problem, and no phone support for merchants to find information for themselves.
There’s currently no ETA for the fix.