Windows is bizarre

OK, so here's what's going on: I attemped installing Debian in the Linux side of my laptop and it completely messed Windows up. I managed to get the bootloader working again but now Windows boots to a blank screen. Thinking it was a driver issue or a corrupt hiberfile, I booted into a installation flash drive to work on everything...and found out why it won't boot when I tried running startup repair. It claims it can't find the hard drive. Ooooook. I've run chkdsk and everything I can think of but no dice. Please help. I can't wipe the drive and reinstall, as I have very important stuff on there.

Since you had very important stuff on there, then surely you must have done a backup of it before messing with Linux on the same computer. I would suggest you try restoring the backup.

It's not clear to me how installing Debian messed things up. Bootloader, yes perhaps. But no Windows detected (apparently), not sure how that happened.

If you have a liveCD/USB then you could boot it and create a summary report and paste the link here so we can take a look at the actual situation.

Note, this does need an internet connection to work from the live medium:
Boot-Info - Community Help Wiki

Again, to be clear, do not try to repair anything, just create a report and post it here.

Hope this helps.

I had something a little similar that might help. I installed Windows 10 on a fresh SSD a few years ago when it was released. I didn't much care for it so went back to using Windows 7, but kept this Win 10 SSD for when I'd eventually swap over. I recently made that move, plugged in the Win 10 SSD, but it wouldn't boot with no boot-manager being found. I found in the BIOS that my data disk (separate HDD) had been prioritised over the Windows Boot Manager on the SSD. Moved the priority to use the Windows Boot Manager and it worked.

I also had need to swap back to Windows 7, so I swapped the SSD over and again, Windows wouldn't boot. This time the BIOS had to be pointed to the drive, which required finding the device in one of the menu's as it wasn't auto-detecting.

And then, going back to Windows 10, again the Boot Manager wasn't detected.

So look in the BIOS at boot and see if Windows Boot Manager is the booting device.