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Did you do anything special with the phone when the screen turned yellow? You can always try to force a shutdown by holding the ON/OFF button + volume up(+) for 10 seconds.
Otherwise, I suggest that you perform a software repair 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.
