Could you better explain in detail what you did before this problem arised. Include the status of your phone while you were doing whatever it was that you were doing? Also, it is asking for a password. This password is done so by the user. Do you not know your password? If you know your password then you should do away with this decryption. Otherwise, i can only speculate that this device was either taken by someone (stolen) or given to you by someone (which I would contact them for that password).
If this device was stolen then I will not help you.
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