A1398 June 2012 not working after battery change

Afternoon all,
Thank you for letting me join your forum.
A customer brought a Macbook Pro Retina A1398 June 2012 model, Core i7MC975LL/A He said he was getting messages saying battery is failing so he bought a new one and fitted it, after this it would not power on at all. I have checked connections and tested the battery and all seem good. When the power button is pressed or the charger is connected, all that happens is the fans power on and nothing else, no start up chime. Tried going back to the old battery and still the same, fans spinning and nothing else.
If anyone can suggest where to start looking i would be eternally grateful.
Thanks guys
John

Here's a great site to learn about Apple products, https://www.ifixit.com. Look for the Repair Guides section, and go through to find the guides for that exact model, size, and year MBP. It could be a cable came loose, or something broke.

What version of Mac OS X/OS X/macOS is on the MBP?

You can try these three things, to see if they will get it working;

https://support.apple.com/en-us/HT204063
https://support.apple.com/en-us/HT201295
https://support.apple.com/en-us/HT201262

Hi Ferrarr,
Thank you so much for the response, I will check the links out to see if anything is there that can help me. I have checked every connection to the board and all are good. I don't know what Mac OS is on it as i have never got it to boot and my customer hasn't got a clue
Thanks again
John

It sounds like the boot drive is not connected. The cables are fragile, so if in replacing the battery the cable was damaged, you would see what you are seeing.