Problems starting mid-2012 MBP

Thanks for your response, HWTech. I've tried booting into Apple Diagnostics, following the directions on the Apple support website. I had to go through the same process of hard shut down, SMC reset, power on, didn't work, hard shut down, PRAM reset, power on, didn't load, hard shut down, etc. before it finally booted while holding down D. It never entered diagnostics, just came to the main page to login. I tried that several times before I trying it with Option + D but that brought me to the same login screen.

I reinstalled the original hard drive in case diagnostics was only in the recovery area of that disk, but that didn't allow me to get into diagnostics, only recovery. In disk utility, it gives me info on the drive. It says Yes to can be verified, and Yes to can be repaired. but bootable is No.

In the past I've upgraded the RAM to 8 GB, and I've replaced the battery. Yes, this is a non-retina model.

Restoring from Time Machine had me rename the restore since the name on the sparse bundle was the same as on my SSD 10.11.6 (El Capitan) install from the bootable USB. I changed the name a bit and continued. I was able to use both users, but the boot up problems remained.

I noticed that the restore from Time Machine that I did a few days ago changed the specs in About This Mac from a mid-2012 to 2009. I tried updating the OSX, but it told me that my computer was unable to be updated to Sierra nor High Sierra when I tried that. When I logged into the user from the new install of El Capitan, the About This Mac didn't have a date in it, not mid-2012 nor 2009. Trying to upgrade the OSX in this user also told me that my computer was unable to be upgraded.

When I had the SSD in the external enclosure, the boot up problems remained.

I didn't try removing the internal drive and leaving the SSD in the external enclosure. It was booting from the external enclosure when it did boot. I've removed the original internal drive and put in the SSD, but it still has start up problems. I had gotten the new internal drive about three weeks ago and put it in an external enclosure in order to get the OS X installed in it. It's now in inside the mid-2012 MBP. It's definitely running quicker with the SSD.

Thanks for your reply. No, I purchased the 13" non-retina MBP in May or June of 2012, and I've had it the whole time. It started with Lion, and came with the Mountain Lion upgrade, which I did. It was currently running El Capitan when the internal hard drive began failing. I kept using it by doing the SMC and PRAM resets, but it kept getting worse. So I bought a new 14" MBP. Just recently someone suggested replacing the hard drive, so I started with that to see if it would solve the boot problems. So far nothing I've tried has made a difference. I will try replacing the SATA cable to see if that has an effect.

I tried to get the computer diagnosed at Apple, but because it wouldn't turn on they couldn't do it. It's considered obsolete. At least I can use the SSD as an external backup. I appreciate all the suggestions. I'll mark this as closed.

Thanks for your suggestion. I installed the SSD in my 13" unibody MBP, but the start up problems continue. I'm going to try upgrading the OS X, but I'm not sure if that will make a difference.