Stuck in boot screen after replacing power supply

The Powersupply was changed today, since I'd learned that the lack of light in the diagnostic LEDs may have been caused by a malfunctioning power supply, while the computer refused to do anything whatsoever (Not even powering on for the boot screen to show).

So, after changing the power supply, it's not able to reach the boot screen (Progress!), but never advances from there. Furthermore I seem unable to boot the machine into safemode (By holding shift on a keyboard after boot sound)

Any ideas, iFixit?