Press and hold the for about 10 seconds to force the phone to reboot.
There are two primary methods to unbrick a ZTE Blade A34: How To Unbrick ZTE Blade A34
Release the when the ZTE logo appears, but keep holding Volume Up until you see the Recovery menu. Press and hold the for about 10 seconds
Sometimes, the system just needs a swift kick. A forced restart can clear a temporary error that's preventing a normal boot. How To Unbrick ZTE Blade A34