OS not booting, but storage is intact?

[Emanuel_12]

An interesting problem I've come across is that my main drive (Win10/boot SSD) has all of its stored filed intact. I plugged it in as an external and can see 100% of my files, programs, and work on there. But the OS will not boot.
Where I think I messed up: I tried installing another version of Windows (xp, if it matters) on a partition on my storage drive (drive 1). My primary Windows 10/boot drive (drive 0) was to be untouched. What I think happened is that installer set up that partition as the actual OS drive (C: in most cases) but ended up damaging/overwriting something on my boot drive. I know very little about how this works, but I know enough to be able to fix it if I knew what the problem is.
I'm thinking that re-installing Win10 on my drive 0 could be a fix but I have no idea. I just really don't want to lose all my files and programs.
What could be the actual problem, and what do you think could be a solid way to fix this?
Thanks very much.

 

[Mr Kagouris]

What exactly happens when you try to boot into Win10?

 

[Emanuel_12]

What exactly happens when you try to boot into Win10?

Essentially nothing. Black screen with error message. I forget the error message but it's along the lines of "Unable to boot from hard drive" or something to that effect.

 

[Emanuel_12]

What exactly happens when you try to boot into Win10?

Follow up: Error message is
"Windows could not start because of a computer disk hardware configuration problem. could not read from the selected boot disk. Check boot path and disk hardware."

 

[LukeFatwalker]

If you have anything attached via a USB, disconnect it. External HDD, iPhone, etc. This error is indicative of storage media interfering with the boot manager and it's boot order.

If you remove all the other storage and it still does it, or you have nothing attached; run Startup Repair to repair your Boot Manager and fix any issues. If this doesn't work, you'll need to re-install the OS. Obviously this is the last thing to try as otherwise you'll lose all of your data.

Either way, recommend backing up your PC once you're back into a bootable state. Either a disk imager (Macrium Reflect Free) or a snapshot tool (RollBack Rx)