15" MBP wont power up after HD upgrade

Hey guys,

I bought a 15" MacBook Pro [mid 2012, A1286] second hand nice & cheap last week cause the display backlight was faulty. I plugged it into an external monitor and it was definitely working fine.

I had bought a Seagate 1TB SSHD to replace the HD that died in my 17" MBP the week before, but decided to put it in the 15" as it's a much newer model.

After installing the new unit in the 15", I put it all back together, hit the power button only to find no signs of life at all [except for the green battery indicator lights on the side]... No fan, no ding, no drive noise, nothing!
I opened it up again & double checked that the battery and all all the other cables/connectors that i may have accidentally bumped were still connected properly. I even tried putting the original HD back into it just in case.
[i've since taken the orig 15" HD out and put it into the 17" MBP to get it working again].

Although the green lights on the side light up when i press the battery indicator button, i tried disconnecting the battery, connecting the charger and still nothing.

Is there any chance i could i have bumped something causing a connection/cable to come loose on the underside of the board?

I'm praying that someone will have the magical cure for this [or at least point out something that i've stupidly overlooked, hehe]... please?!

 

I always make sure to touch something metal, etc...

The cat rubbing itself up against my leg wouldn't create enough static to hurt it, would it?