Factory reset sony ericsson u5i

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

All data including third-party applications will be deleted from the Sony Ericsson Xperia X8. Once the wipe is complete, the phone will reboot to its factory fresh state.

The password is user defined, try the second method if you did not set a password.

Good Luck!