From the Neopian Times Issue #295:

Q: Hey, what's happened to the Neopian Bank? It's been down for two days now and I can't find any info as to why it's shut down or when it will be back up.

TNT: We're very sorry about that. There was a glitch that was discovered by some players that would cause Neopoints to double. Some players horribly abused this glitch, making billions of Neopoints, then proceeded to attempt to inflate items in the economy. To stop the damage to the economy we had to take down the Trading Post and Auction House, along with the bank. The players that maliciously used the glitch are being tracked down and frozen so they can no longer harm the economy and ruin the site for everyone else. Rest assured that, if you didn't abuse the glitch, your Neopoints are safe and will still be in the bank when it's back up and running, though we don't believe we can compensate for lost bank interest. We're very sorry that the actions of unscrupulous Neopians have dampened your Neopets experience for the past few days. :(

Please note, also, that the glitch was caused by something we did on our side, so despite anyone trying to "take credit" for the bank glitch, we're afraid it was our bad. :X

The players that maliciously used the glitch are being tracked down and frozen so they can no longer harm the economy and ruin the site for everyone else.