Google recently created its 'Ad Manager' brand from two existing products, Doubleclick for Publishers (DFP) and Ad Exchange. If you are trying to visit Google Ad Manager in your browser and all you get is the following symbol...
...then it is worth checking to see whether you have Adblock Plus (ABP) installed. Certainly in Chrome, this extension prevents me from loading or using Ad Manager.
I'm not sure whether it is ironic or simply logical that an ad blocker would stop an ad platform from working. And it is easily resolved: either disable the ABP extension entirely (in Chrome, go to chrome://extensions/), or disable it for Ad Manager only.
To do the latter:
I'd be interested to know if other ad blockers have a similar effect on Google Ad Manager.