Update: Official Gmail Blog has confirmed that the outage happened and is fixed. The problem was caused by the contacts system used by Gmail.
Many of our Twitter followers started tweeting, that Gmail has been down for them.
We could confirm that Gmail is down for one of our accounts, but another account worked fine which means that this problem is only limited to certain accounts.
For the affected accounts Gmail displays a message as shown in the image below.
Is your Gmail account facing a problem too?