Here’s the problem in Apple’s words:
There is an issue with Apple TV software version 3.0 that can possibly cause your content to disappear after a period of time. All customers running Apple TV software version 3.0 should immediately restart their Apple TV and then upgrade to Apple TV software version 3.0.1.
Today, Apple has sent an email to all Apple TV owners advising them to immediately upgrade to the new 3.0.1 software to cure this issue. Obviously, you’ll want to do this as an Apple TV with no content is pretty much useless.
The new Apple TV 3.0 software has been generally well-received, though many feel it doesn’t go far enough to allow for content such as streaming web video. The UI has been improved, but it’s still hard to search for content without using an iPhone or iPod touch as the remote. The new Apple Remote works with the device, but it’s pretty much as useless as the white one in many ways.