If you are experiencing issues with your O2 XDA Zest then a hard reset may be the way to go. Always make sure you backup your contacts and data before doing a hard reset back to factory. Read below for how to do the Hard-Reset or Soft-Reset.
O2 XDA Zest soft reset
Insert your stylus into this hole to soft-reset /restart your Xda Zest.
O2 XDA Zest hard reset
Warning! All your data will be deleted. Backup all necessary data first! Press navigation UP + press the RESET hole