First, try this:
1, From the main home screen, tap menu
2, Tap Settings, then Privacy
3, Tap Factory data reset
4, Tap Reset phone
5, Enter your password if required, then tap Erase everything to confirm
If that doesn't work, or if your phone is frozen:
1, Turn the power off. If your Sony Ericsson Xperia X10 is frozen, pull the battery out and reinsert it
2, Hold the Left and right buttons located beneath the display
3, Press and release the Power button
4, Follow the on-screen instructions to hard reset your Xperia X10
Click to expand...