Bootcamp Assist Disappeared after Windows Update
Separate to the Boot Camp Control Panel, you can always choose your startup disk by holding alt/option on boot.

Shutdown, then hold alt/option while tapping the power button until the Startup Manager appears.
Choose your macOS startup disk to start from macOS this time.
Download the support software from Boot Camp Assistant and copy it to an external disk or other shared location between Windows and macOS.
Restart your Mac, which will boot into Windows again.
Reinstall the Boot Camp support software you downloaded and copied in step 3.

If you want to use Startup Manager in the future, something perhaps useful to know is you can change the default disk from the Startup Manager by holding control while choosing your startup disk.