my pal has started having this problem with his 360 and i have no clue what is going on maybe some of you can help!
his xbox will turn on streight away with no hassles the first time he switches it on and will play for hours without a glitch but as soon as it is switched off if you try to switch it back on straight away it wont.
the power light comes on but it wont boot and the screen stays black it doesnt matter how many times you switch it off and then back on it just wont have it this is the same if you physically switch it off and back on or switch it off from the dash and back on with the controller.
if you leave it off for five minutes untouched it will switch back on immediately.
it isnt a big problem as it isnt offten you need to switch off and back on again but its a little annoying as you know somthing is wrong just not what it is.