Welcome to Vodafone Community
Can you give this a try for me.
Long press 'upper side key' and 'power on key' at the same time, under power off mode. You will then see 'All data will be lost, continue?' After you select 'Yes,' the sequence will be entered with reminder 'Formatting flash, please wait.' The phone will auto switch on when reset finishes. Please do not manipulate the phone nor remove the battery during this period of time.
Let me know how you get on.