Manually enable MagiskHide in settings (MagiskHide is no longer enabled by default). Also, there are known methods to detect Magisk, so your mileage may vary.
If you have USB debugging enabled in developer options, connect your phone to the PC. If your device is detected (check by
adb devices), enter ADB shell and run the command
magisk --remove-modules. This will remove all your modules and automatically reboot the device.
If unfortunately you do not have USB debugging enabled, reboot into Safe Mode. Most modern Android devices support pressing a special key combo at boot to enter Safe Mode as an emergency option. Magisk will detect Safe Mode being activated, and all modules will be disabled. Then reboot back to normal mode (the module disable state persists) and manage your modules through Magisk Manager.
On devices running Android 9.0+, Magisk Manager will use a more advanced hiding method. It will install a “stub” APK that has nothing in it. The only functionality the stub app has is downloading the full Magisk Manager APK into its internal storage and dynamically load it. Due to the fact that the APK is literally empty, it does not contain the image resource for the app icon.
When you open the hidden Magisk Manager, it will offer you the option to create a shortcut in the homescreen (which has both the correct app name and icon) for your convenience. You can also manually ask the app to create the icon in Magisk Manager settings.