At first I had thought that the service outage was the work of Chinese hackers. Later I realized it was due to my service provider Yahoo!. In February, I upgraded my service plan to unlimited disk space and unlimited data transfer. But I did not know there was a catch: Yahoo! has the right to determine how fast my site can grow. In other words, data transfer is unlimited only if Yahoo! pre-approves it. So when the data transfer amount exceeded the limit around March 20, the service was disabled.
The error message asked the owner of the site, which is I, to contact the customer care. I wrote twice to them to complain about the limited unlimited data transfer, but no one has ever replied to me. After all, I only pay less 12 USD per month to them and will never be qualified as their valued customer.
Anyway, I apologize for the inconvenience during the service outage. Please come back often. But not tooooooo often before my data transfer allowance becomes truly unlimited.