It is unlikely that any phone gets hanged because of too many apps running or some apps could make your phone hang. In that situation you could simply remove the battery from the phone and put them back and start it which will work fine. But on the Moto G5 Plus there is no option of removing the battery from the phone as it is sealed.
So press and hold the power key for about 10 seconds which will restart the phone. You don’t have to worry about the data loss as it will not make any harm to the data on your phone. You could check the data when the phone gets started. Also make sure the phone battery level is at least 10%, else the device will not turn on. In that situation put your device on charge and allow it to charge for some time and then try press the power key which will turn the device on.
If you are still unlucky as the device doesn’t start up, here are some steps for the hard boot.
1. With the phone powered off, press and hold the Power and Volume Down button at the same time until the device turns on. Use the volume up and down to move between the options and power key for selection.
2. Press the Volume Down button to highlight “Recovery mode”
3. Press the Power button to restart into Recovery mode.
If you are aware of the other options like wipe data/factory reset you could try them as well.