If the phone is "soft-bricked" or the button combinations aren't working, you can use the Xperia Companion (formerly PC Companion) on a Windows computer. How Do I Factory Reset with PC Companion? - Community
This is the most common "hard reset" method used when you cannot access the phone's menu or have forgotten the screen lock. : Turn off your Sony Ericsson MT11i completely. sony ericsson mt11i hard reset
: While holding those two buttons, briefly press and release the Power button . If the phone is "soft-bricked" or the button