ZTE Blade A3 (2019) - How to soft reset

To soft reset your phone press and hold the Power/Lock button for around 8 to 10 seconds. The phone will restart automatically.

If that doesn’t work, then remove the battery (if it has one). Wait 30 seconds and put the
battery back in. Then press the Power/Lock button to turn the phone on.

Some devices also provide reboot option on pressing Power Button.