*Bad flash - either momentary issue with usb connection between phone/PC (or even an issue with the USB board on the phone), or pulling the cable in the middle of a flash, or flashing wrong firmware.
*Simple component failure on the motherboard, either the EMMC itself or some other component. Fact - electronics breaks.
Regardless of cause, if you want this fixed properly, you replace the motherboard. Some unscrupulous repairers may try to sell you a fix that involves replacing individual components on the board, but that's very very difficult even for highly skilled people to do, and would probably cost you more than a motherboard replacement given the time involved; having it done by someone actually qualified it would cost more than a new board, put it that way. Someone not particularly skilled/qualified might charge you a lot less, but the repair will quite possibly fail, and you'll need to replace the board anyway - and you get charged for the failed repair plus the cost of replacing the board. You also run the risk of having other components in the phone damaged by someone with less than stellar skills.
I have never seen anyone on here post they've had individual components on the motherboard replaced successfully.
Motherboard replacement cost varies widely from country to country mostly dependent on cost of labour - if you want a new board, talk to a Samsung authorised repairer (expensive option). Or you can either source a 2nd hand board yourself online & pay a local mobile repair shop to swap the boards (cheaper option), or they may be able to source a 2nd hand board for you.
So, bottom line - if you want the phone fixed without fuss guaranteed (unless there's problems with other components), you replace the board & don't even consider anything else.