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Hold the power button longer for the options
Try a reboot - Hold volume up and power for around 10 seconds
Edit: Then when powered off start your phone again and test
The only thing I can think of is to perform a system repair
Switch off your phone and unplug from Pc (Hold volume up and power for around 10 seconds)
Start PC Companion and select Support zone then Update my phone/tablet then in Blue Repair my phone/tablet and follow the onscreen instructions - When prompted connect your phone still switched off holding volume down - This should start the repair or reformat process
If you are using Windows 8/8.1 or a 64bit operating system then adjust the settings for PC Companion and run in compatibility mode and chose Windows 7 or XP
Budle of applications sometime makes the phone volunerable...so the last option left always is RESET...always try it but in the END.