Hanging on BIOS splash screen after upgrading

I am now at my wits end having just upgraded some of my hardware.

I recently bought myself a new WD 2Tb sata drive, a new midi tower case and a Gigabyte GeForce 970 GTX card. As my old tower couldn't take the new card, i needed a new tower that could.

I spent Thursday stripping out the PSU, mobo, soundcard, hd drives and RAM and rebuilding them in to the new rig. Apart from some swearing when i realised the ATX cable literally only just made it to the socket with some strategic manoeuvres, the new build was ready to go. I hit the switch.... and everything stopped at the bios splash screen....

I hve read through some of the other threads here for any help and nothing worked. I've reset CMOS, i've taken out the battery, i've stripped everything out and reseated all connectors, i've tried each device individually. The speaker is sounding the single 'everything okay' beep. I take out the RAM and reboot and the speaker goes mad, as expected. The keyboard is working fine as it allows a ctrl/alt/del reset, but won't let me in to the BIOS.

I was about to put the original rig back together to see if something went wrong between the two, but one of my lovely kids spilled a glass of orange over my old GFX card....

Help!

Current unworking build;
Gigabyte GA-Z77-DS3H
Intel i5 6400
4Gb DDR3 Corsair XMS3
1Tb Seagate Barracuda
2Tb Western Digital
Gigabyte GeForce GTX 970 Windforce
Inwin GT1 Midi ATX case

Turns out my PSU had some issues nd was not putting out, so my sparkly new GTX was left disappointed and unsatisfied.

(My supply is 750, card reqs said min 500)