i bought a phone but its not functioning. the seller said that it is but its not

[de_abhin]

i bought a used phone from a seller but its not functioning properly. The seller said that it what functioning properly When He sent it but just after using it for an hour its not functioning at all. I've received the package yesterday and since then I've been having this problem. what can i do now ??
I've filed a case and I'm in contact with the seller too but he's denying the factthat its not working and it was a private auction so I'm in a problem right now. as you can see in the picture below, I can only use the space highlighted on the screen and can not even open WhatsApp or walkman etc.  can u please help me out. Thanks in advance