An R9-390X is quite an old card... how long have you been using it for mining? It honestly sounds like running it at 100% load while mining has probably overheated and killed it... maybe a VRM has let go? Do you see any discolouration on the back of the PCB near where a chip is soldered?
If the card is not working when plugged directly into the motherboard, that would rule out a bad riser etc. Your card sounds like it has become a paperweight.