Iphone X is completely died !!

Hello ..

I've bought my iphone X as a used device 5 months ago .. Everything was going fine .. The iphone was working smoothly without any problem with battery or software or etc , ...

yesterday and suddenly while I was using the device .. it suddenly turned into a black screen and shut down .. no response to any action like its dead

I've tried force restart .. tried to leave the device on the charger for the whole night with no response .. it does not even show the battery icon

I've tried to connect to iTunes with no response .. even the computer does not recognize USB connection

My phone is totally dead and I am so worried ..

what can that problem be ?? is it the battery ?? or something else ??

btw my battery health was 87% and it was working very fine

please help