I had that happen to me with a
Blemished Base Model MPC 4000 i bought from zZounds. It happened on the 3rd day i had it too. I should've known something was up when the one i received from them was "white" and not "blue" (they only advertise having the blue ones). I wound up sending it back to them and paying the extra $450 to get the New Base Model. But during the time i had the Blemished Model, i called the Akai Support Team and they told me that the Flash Rom was either bad or going bad and needed to be replaced
not fixed! The screen read
Flash Rom Failure: Code Is Invalid! and all of the function lights on the board were lit up, what a nightmare! You have to send your 4000 to Akai to have the Flash Rom replaced, no one else provides that service.
Another note on my "Flash Rom Failure" experience. I tried reinstalling the OS with the OS Loader and then rebooting. The screen would never leave that "Flash Rom" page. I tried installing all the different versions of OS to see if that would work and it never did. I finally cut the machine off, left the house for some hours, came back, turned it on and the machine booted up. I used it that night with no problems and turned it off when i was done. The next morning when i woke up to turn it back on, that "Flash Rom" message was there and this time, it never left! Hope your MPC's still under warranty - sorry to hear about your mishap, i know how it feels!

I come from the Cold School...