PC not properly working after 'Grey Screen of Death'

[Background]

While gaming, the game client had malfunctioned leaving me unable to force quit the program (or do anything else) with just a black screen. I force shutdown my PC by holding the power button and turned it back on. After a long load time on startup, I encountered what I found out was 'The Grey Screen of Death,' so I force shutdown my PC again since I didn't know what was happening. After another long boot up, there was bottom text saying "Scanning and repairing drive (D:): 100% complete" just before it switches to the pin lockscreen.

[Issue]

Logging in, I found that my (D:) drive is just missing (the icon shows, but the info and storage bar are not visible). So now I have no access to said drive, and my PC has significantly slowed down when shutting down, restarting, and starting up. No videos or forum posts have helped so far, so any advice?