After upgrading to High Sierra, Boot Camp partition is gone from Startup Manager
I do not yet have enough information to diagnose your problem. However, I can offer an solution to one possible scenario that fits what you have posted.

I assume you started with a previous version of Windows, say Windows 7. At some point you upgraded to Windows 10. If this is true, the you would have been using the legacy BIOS boot method, which requires a hybrid GPT/MBR partitioning scheme.

It is possible the macOS upgrade converted a Hybrid partition scheme back to a pure GPT scheme. In this case, you problem is a duplicate of How to convert a drive from the GPT format to the hybrid GPT/MBR format when using High Sierra (macOS 10.13.2).

Before implementing any solution, I would start by determining whether your Mac is trying to boot Windows using the legacy BIOS or the newer EFI boot method.

This can be done by executing the following steps.


Open "Startup Disk" under "System Preferences".
Unlock if necessary.
Select the icon labeled Windows. Do not restart the computer.
Quit "System Preferences"
Open a Terminal application window.
Enter the command given below.

bless  --info  --getBoot  --verbose  2&gt;&amp;1  |  grep  -i  legacy

If you are using a legacy BIOS boot method, then the output should appear similar to what is shown below.

Legacy mode suppported
Boot option is a legacy device
Searching for legacy type 'HD'
Legacy mode suppported
Legacy mode suppported
Matching legacy device 'disk0s4'
Legacy boot device detected


If you are using the newer EFI boot method, then the command will produce no output.
Repeat steps 1 through 4, except select icon with label indicating the volume you have currently booted from. 


If you post the output from the the command shown in step 6, I can update my answer with a better explanation on how to proceed.

Also, the output, from the command given below, can help diagnose your problem.

diskutil  list