In my experience, if fan turns up and then powers off and nothing on display, only leds shortly on: [br]
Most likely problem: memory or CPU[br]
Start with removing all memory and reinsert just one memory module and try booting again[br]
if still not booting: try to boot with every memory module separatly to find out which one is broken and which isn't[br]
if the problem remains with all memory modules: most likely cause: CPU problem