Hi,
Firstly, just try charging the phone. If this works, how much charge is in the battery? If low, fully charge, then try to turn on phone.
If this fails, try connecting the phone to a PC. See if it works. Hopefully it does, so that you can back up all your phone data to the PC in case in freezes again.
As a last resort you can try a hard reset
'''WARNING! Master resetting your smartphone will result in the deletion of all personal data stored on the device, including downloaded content, images, custom ringtones, applications, contacts etc. All settings will be restored to factory values. Data stored on the SIM and SD cards will not be affected.'''
Turn off your smartphone
Press and hold the "Volume Up" and "Power" buttons at the same time
When the "ZTE" logo appears on-screen, release the keys
Press "Volume down" to navigate to and highlight "wipe data / factory reset"
Press "Power" to select the option
Press "Volume down" to navigate to and highlight "Yes -- delete all user data"
Press the "Power" key and wait for your smartphone to reboot
if you don't wish to do this, give ZTE support a call, they may have a less drastic option