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just press and Power & volume up for 5 seconds, make sure that you are not pressing any other key/volume down
Don't forget to give Kudos
Just follow my suggestions
Press and Hold POWER & VOLUME UP at the SAME TIME for 5 seconds until the phone vibrate.
Is there anything holding/pressing the volume down key? do you have a protective case? if you do, remove it and try again
How long do you press the power button? do you press it only until the phone vibrates or do you keep pressing/holding it?
This might be helpful
http://talk.sonymobile.com/t5/FAQ/How-to-boot-your-phone-into-safe-mode/m-p/348008#U348008
Don't forget to give Kudos