Am I missing something obvious with this HTPC chassis? At this point, I can only think of getting a different motherboard to see if that's the problem but the fact everything is getting powered makes me think the motherboard might be ok. Does this kind of case make any beep for the motherboard as it powers up?
Please let me know if there's any more information I should add...
Oh and I should mention there's no information displaying on the front from the LED readout
Yes, both the 4-pin CPU power connector (known as ATX for this motherboard) and the 2x12 Main Power Connector are in.
I've switched out the RAM and still nothing.
It's so weird, I've never had a box I've built NOT boot up.
The CPU and it's fan are in, but shouldn't it at least boot even if the CPU is bad? If the motherboard is bad, should it still be able to power the fans?
Were you originally trying to boot without standoffs installed between the motherboard and case? If you were, I hope you didn't fry the motherboard. The fact that the fans, lights, etc... light up DOESN'T mean the motherboard is fine.
Have you checked Gigabyte's website to see if that particular CPU is supported on that motherboard?
The Antec manual says that standoffs are already installed. It's not just the plain chassis, there are standoffs already mounted inside.
I got some extra brass standoffs just to make sure.
I first tried booting without the extra standoffs installed so perhaps that's it for the MB. With the extra standoffs in, the MB sits up high and doesn't look right. There are some already in the chassis but they don't look brass.
I got some extra ones and added standoffs to the standoffs? just to see if that might be the problem.
The CPU is supported, I've an Athlon 64 AM2 which the MB supports
There's a lot more to a CPU than the series (Athlon 64 AM2). Something as minor as a new stepping will often require a new motherboard BIOS for support. Just because your chip is an "Athlon 64 AM2" doesn't mean it's automatically supported on your board. What is the exact model of your motherboard? I went to the Gigabyte site and looked at all the 780G boards listed and none of them showed the 5400B in the supported CPU list. That chip looks like it's a "business class" chip, whatever that means.