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If you have forgot the PIN-code for your SIM-card, I suggest that you contact your operator for further assistance in this matter.
But if you have forgot your PIN-code to your screen lock, you need to perform a software repair on your unit via PC Companion:
http://www.sonymobile.com/global-en/tools/pc-companion/
In PC Companion you press start on Support Zone > Start on Update the phone/tablet software > Repair phone/tablet (the blue clickable link in the message) > Follow these steps without having your device connected to the PC.
Follow the instruction that appears in the program and do not connect your device until this guide tells you how to connect it.
