Question mark folder after successful SSD/RAM upgrade

Mid 2012 MacBook Pro

Upgraded HD to PNY brand SSD 240 g

Upgraded RAM to PNY 16 gig

Installed High Sierra successfully and it was running REALLY fast(compared to how it was).

I opened Safari, Facebook etc and when I went to click out of a window BEACHBALL! Computer was good to go for a few hours.

Force quit the Mac and when I turned back on, nothing but folder with obnoxious question mark staring back at me like a Nietzschean abyss. I tried to reboot holding control R or whatever, but I’m pretty sure I need a Bluetooth mouse now(even more $ to sink into this jalopy of a machine) because when the disk utility boots the mouse is frozen and will not click or even move. How convenient.

I ordered a new sata cable for the HD as I’ve read ad naseum that this is something to replace anyways. I’ve put so much money into this ancient thing that I should have just bought a new one. Should I just take it to the over priced Apple/PC repair guy who told me it would cost me $300 in labor to install the new HD/RAM? Or should I keep the faith and continue to blindly try to figure this out?

Pro tip-I’ve never done anything like this before.

Thank you in advance for any input or solutions.