Make sure its enabled in bios first & check in device manger that both are detected might be a fault in your bios with your motherboard drivers out of date or corrupted not picking them up. Another thing couald be a faulty mobo something failed of a Pcie lane has dropped or failed can happen. Another idea is shut down your system pull both cards out blow out the pcie slots blow out both cards press them both in firmly repower up to bios & enable crossfire in your bios & see if both are detected when loading into windows threw device manager first.
