Hard drive Failure or corrupt boot sequence?

guys, I hope someone could help me with this as I been pulling my hair out these past few hours.

I was playing a PC game earlier today and a few hours in the screen freezes completely and a loud buzzing sound kept looping. I couldn't cntrl alt delete out of it and had to restart via the power button. I come back on and try and play again and the same thing happens, but this time while I was browsing on my desktop with the game minimized.

After this crash however, I get a black screen on boot up with just the words "missing operating system" on top.

I tried using my windows 8 installation disk to repair with the system refresh and startup repair, but neither of them worked. The first option kept saying my drive was locked and the second didn't give a reason, just said it couldn't fix it after running for 3 hours.

I tried fixing the locked drive message by using all the command prompt options available off here and other google searches but that didn't work.

I am starting to think my hard drive is dead. It's about 4 years old now and I've heard that's a lot give out around then, especially since my computer is a gaming rig and runs pretty hot.

Before these crashes, my roomate was using my computer and said navigating the desktop was extremely slow. I also notice my disk usage was at 100% even when I didn't have any programs open. My hard drive fans also seem to be running unusually high these past few months. I never really paid any attention to it though as I never heard any strange sounds coming from it (sounds fine) and the slowdown issues only really became apparent earlier today.

I am not entirely sure though if it is my hard drive as this only happened after I accidently knocked my graphics card video connector loose mid game. The slowdown occured before the game, but I am not sure if forcing power off twice caused a corruption on my boot file. I don't know too much about software so I don't want to jump to conclusions as I will feel a bit silly replacing my entire hard drive and OS if it's only a corrupt file or something.

I have tried that already, wasn't able to work