It looks like Google have disabled existing keys until your agreement is updated - I had to turn on the Maps API v3 service, then my browser key for the site started working again:
Visit the APIs Console at https://code.google.com/apis/console and log in with your Google Account.
Click the Services link from the left-hand menu.
Activate the Google Maps JavaScript API v3 service.
I already had a browser key created for that domain from back in February, and that key now works again.
If you haven't already created Simple API keys, create a Simple API Browser key for your site in your Google console. It should look like this:
Key for browser apps (with referers)
API key: xxxxxxxxxxxxxxxxxx
Referers: *yourdomain.com/*
Activated on: Feb 28, 2014 4:13 PM
Activated by: xxxxxxxx@gmail.com – you
Note: You don't need to create a key unless you have a lot of traffic and a lot of maps usage. If you exceed 25000 requests per day for more than 90 consecutive days you will be exceeding your limits and will need an API key to continue service as normal.
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updated by @ultrajam: 10/17/14 10:20:16AM