That's weird.
It either is, or isn't connected with your change from UEFI to Legacy boot mode.
If it is, then the problem is likely to be a configuration issue, either in the card, and/or on the motherboard. can you reset both of them completely?
If it isn't, then it may be a hardware failure. I would try another card first. If that works, you know there's something up with the card. If it doesn't work, then suspect the motherboard. At this point you could try probing the supply pins on the interface, or even the 12V rails on the power supply (and indeed *all* the rails on the power supply)
Be careful probing because the pitch of the pins on the interface is small and shorting pins could easily toast your motherboard.