take it back.
They may tell you that Apple has responsibility, the warranty, but you may be able to talk them into taking it back.
Get on the phone to Apple and explain the problem to them.
Do it quickly as Apple does not like DOA and the longer you wait, the less likely you are to get a replacement rather than yours repaired
I have a brand new iMac 24" screen, a week old. The computer will not start up. It worked for two days, then shut itself down. I pushed the start button, would not boot. I unplugged, let it sit overnight, I pushed start button, it started up. I left it on for a few hours, then tried to restart, it would not boot again. The next day, it started up, i left it alone all day to return to find a blank screen and it will not respond to mouse or keyboard. I have unplugged again, then relighted, will not boot! Is there a fix to this or should I just return it??