After El Capitan install screen shut down, now won't startup

Hello everyone, new to the ifixit. Never had issues before this update, not glitches. So after updating to El Capitan, almost immediately, if I played a video and scrolled through another document my screen would shutdown, so I had to do a manual restart eventhough the computer itself seemed to be on, the screen was blacked out.

I stopped watching videos to avoid that. A week later the update to are Capitan came out so I updated. I still avoided videos while working on the homework.

After the second update, I couldn't even scroll through any kind of document because again the screen would shutdown.

Finally, the computer would not start. It went to the apple logo and status bar and right before I gets to load completely the screen shuts down. I've tried reinstalling from safe mode, but now when the El Capitan asks me to select the HD to install to two things happen, either there is no HD to select or it is locked. I tried 'journal enabling' from the disk repair, but the option is greyed out.

I've contacted apple and they told me I probably need a new hard drive or a new 'cable.' I refuse to believe my computer crapped out over a software update, but it worked just fine before the update. Never had an issue. Apple is saying that this was was probably gonna happen anyway, but there was no way to tell.

Help, please, I have midterms.