flashing folder, is it the HDD or cable?

Hi, I have a MacBook Pro 2012, 13"

When I turned on my computer the screen was grey and a folder with a question mark started flashing.

I have tried (in no particular order): resetting the SMC, connecting it via FireWire to another MacBook Pro to try & pull my files, repairing the system folder + a couple of other things.

It doesn't seem to be able to recognise a system on the computer, but when I go to Disk Utility there is a Base System which it says has no errors on it.

I'm trying to find a way to solve the problem myself as Apple want to charge to get online support and there are no available appointments at any of the first 7 apple stores that have a Genius Bar... From reading other threads I'm wondering if it is the hard drive or possibly even the cable?

I'm not technical, so what sort of cable connecting to the hard drive should I be looking at purchasing? I have found one called 'MacBook Pro unibody 13.3" A1278 hard drive HDD Cable, 821-0814-A" on Amazon - does this sound right?!

I assume this should be the first step before purchasing a new hard drive altogether?

I appreciate any help you can give!

Ash