NOTE: "Performing a factory data reset deletes all data, including downloaded applications, and resets the device back to its initial state—the state before you turned on the device for the first time.Important Make sure to back up important data you have on the device before you perform a factory data reset."
HTC Tattoo Hard Reset Procedures
1. With the phone turned off, press and hold the HOME and BACKand then briefly press the END CALL/POWER button.
2. Press the ENTER button to begin the reset process.
Tip If your phone hangs or freezes, remove the battery then wait for a few seconds, and then re-install it. After re-installing the battery, turn on the phone.