Due to a broken fan retention module, I had to take the motherboard out of the chassis. After connecting everything back, everything seems to work (fans and HDs are spinning, network adapter's lights are flashing, etc...) but I get no display. I tried two different display adapters, tried disconnecting all other expansion cards but nothing seemed to work.
Update: just noticed that some of the usb connections aren't getting any power as well (after double checking, it seems that different devices seem to be giving different results regarding that one - mouse has its light on, wireless network adapter does not).
Some more updates: After some investigation, the situation looks like this - after power on, all fans are spinning, HDs are spinning, and CD-ROM is looking for media. However, it doesn't look like the OS is loading, since other than some flashing at the first few seconds, the HD activity led doesn't report any activity (the HDs are definitely spinning). I'm start to worry that perhaps the CPU was damaged before all this process began (as the cpu fan retention was broken), but it usually powers off on high temperature.