Were you having this issue before you upgraded to Mojave? Also, what Mac OS were you running prior to upgrading? And did you upgrade "in place", ie, basically let Mojave overwrite what was on your Mac already?
Also, I copied this from a link that I found via a search for "How to switch Between Windows and Mac OS":
"SWITCHING OPERATING SYSTEMS
You can go back and forth between OS X and Windows on your Mac, but you can’t run both operating systems simultaneously under Boot Camp. Instead, you have to boot one operating system or the other — thus, the name Boot Camp.
Restart your Mac, and hold down the Option key until icons for each operating system appear onscreen. Highlight Windows or Macintosh HD, and click the arrow to launch the operating system of choice for this session.
If you want OS X or Windows to boot every time, choose app → System Preferences, click Startup Disk, and choose the OS you want to launch by default.
You can perform the same function in Windows by clicking the Boot Camp system-tray icon and selecting the Boot Camp Control Panel. Click either the Macintosh HD or Windows icon, depending on your startup preference."
So, according to that, when you start up your Mac and hold down the Option key until icons for each operating system appear onscreen. Also, it looks like you can do something similar in Boot Camp.
If neither of those work, but were working with the prior Mac OS you were using, then it would indicate some issue either in Mojave itself, or with the way you upgraded.