I tried to play AC2 on an Xbox360, applied the update to the console (as directed) and the game started freezing up moments after starting play. Now my xbox360 will not start up at all.
you got the RROD? 360's love to fail.
check you don't have 3 red lights on the front round the power button if you do your console is dead.
check your power supply lead make sure the green light is still on it if this is red your power supply is broke, if there is no light at all check the connection to your console and change the fuse in the plug and see if it works.
the second solution can fix a RROD as it can be caused by a faulty power supply, if your power supply is still on (green light) and you are just getting no response from the console the console is dead.
either way the it wont be something you can fix your self, unless it is the power supply then you can just buy another but make sure it is the correct one for your console as they have made several versions to coincide with different hardware revisions in the 360.
but more likely than not you will have to send the console to Microsoft.