Can't boot Windows after installing Windows 10 with Boot Camp
This article might help: https://support.apple.com/en-us/HT204417

Section below:

Use these steps to choose a startup disk with Startup Manager:


Turn on or restart your Mac. 
Immediately press and hold the Option
key. After a few seconds, the Startup Manager appears. If you don't
see the volume you want to use, wait a few moments for Startup
Manager to finish scanning connected drives. 
Use your mouse or
trackpad, or left and right arrow keys to select the volume you want
to use.
Double-click or press the Return key to start up your Mac
from the volume you selected.