That happened to me 480dork, except my startup screen is Nakamichi, not AKG
The first time it happened I held the power button in for almost 30 seconds, and it shut down. Turned it back on, everything was fine.
The second time, I tried the same thing, and nothing worked. I pushed all the buttons, hooked it up to my USB, everything. I even tried pushing the reset button, and that didn't even work.
So I took the batteries out, and everything was lost, including a 2 hour show and a copule mp3's of my band.
Sorry about this! Hopefully I can help a little.