Well, yes really. It is possible to do a number of things in windows beforehand to make it work with another motherboard, but as this isnt an option for you, you will probably have to reformat.
Of course, if you get the same motherboard (brand and type) you shouldnt have any problem and no need to reformat. I cant remember when you got it all, but shouldnt it be under warrenty?
Its possible it isnt the motherboard, so be careful. Its not the monitor, as I assume your using that now...
I dont know what to suggest really, apart from change the motherboard.
I hope it is the motherboard, maybe someone who knows more than me can post.
Either way, good luck, and custom pcs are not usually this much trouble! You seem to be very unlucky. :shrug:
Oh, a quick thought. If you cant even get into the bios, its very likely it is the motherboard. Have a quick visual check tho, just to see theres nothing obvious. Like for example, the psu went funny and fried the motherboard. This can happen with power surges, and you can sometimes see damage.
If you can be bothered, you could try the psu your using now in it, just to eliminate something else.
I always recommend a power surge adaptor, with both pc and monitor plugged into it. Always better to be safe than sorry.