OP --
Pay
much attention to Bluedog's reply.
The old drive could still be fine -- but the 2012 design non-retina MBP's have a reputation for having the ribbon cable (for the drive) go bad.
When it does go bad, proper communication is lost between the drive and motherboard. So... the computer can go "all wonky" (slowdowns, failed boots, crashes), or... just won't boot at all.
The easiest way to determine this is to spend $10 and buy an EXTERNAL USB3 drive enclosure.
Something like this:
https://tinyurl.com/y4mj6gzg
(had to shorten the URL)
Put the drive in and connect it.
Then press the power on button and hold down the option key to bring up the startup manager
Select the external drive and hit return.
Do you get a good boot from the OLD drive?
If so, then it's the ribbon cable.
Go to ifixit.com to get the part number and see how to change it.
You can buy from them or order elsewhere online.