If this image brings back bad memories...
Or you get the "Black screen with a blinking cursor." That means there's a problem with the boot loader on your Windows install. The good news is that this doesn't mean the end of the world for your Windows install! The bad news is you're going to need to get your hands dirty in the command line. And there's also two paths you can take and I'll go through both.
Using a Recovery Environment
The only requirement is that you have a Windows 10 install disk or USB drive. Microsoft provides a tool to get the ISO so you can either burn it to a disc or make a bootable USB from at https://www.microsoft.com/en-us/software-download/windows10
Now then, this is the best case scenario: you just need to fix the boot loader.
Boot into the install media
When you get to this screen, select "Repair yourcomputer"
Select "Troubleshoot", then "Command Prompt"
Then enter in the following:
If this doesn't work, you'll need to create a new partition for the boot loader (Note: If you already have a bootloader partition (it should be a few hundred megabytes), you can skip to step 4). Shut down the computer, unplug all storage drives except the one Windows is on, then power the computer back up.
From the last bit, follow until Step 3 then enter in the following:
select disk 0
create partition primary size=200
This should create a small partition
Verify a partition of 200MB has been created (this example created a 500MB partition). Note its number
select partition 2
Or whatever number the small partition is
format fs=ntfs quick
Verify that "Active: Yes" shows.
bcdboot C:\Windows /s S:
This assumes that C: is the Windows partition. Verify which drive letter the Windows partition really is. If successful, you should get the following:
Using another computer
If you can install the disk on another Windows computer, it'll be a bit easier to work with. However, this method only creates a new partition. I believe the bootrec.exe option may muck with the computer's boot settings, not yours.
Since I'm feeling lazy and I didn't really prepare for this one, I'll just copy this from a page I reference often:
Use Disk Management console to create a new primary disk partition from unused disk space for the new System partition.
Go to Control Panel -> Administrative Tools -> Computer Management -> Storage -> Disk Management
Use 200MB of free space on an existing partition and shrink the partition.
Create the new primary partition, and format using NTFS.
Assign the drive letter, S:.
Mark the new partition as Active.
Right-click on partition S:, and select Mark partition as Active.
Use bcdboot.exe to copy the Windows boot files to the new System partition.
Execute the following command: Bcdboot D:\Windows /s S:. Bcdboot copies the required boot environment files for your Windows install to the S: partition, and creates a new BCD store to boot your Windows install from the D: partition.