LG 5K monitor no longer detected (still powers Macbook Pro)

LG UltraFine 5k monitor no longer detected by MacBook Pro. I have used this configuration for years without a problem. Suddenly it has stopped working. Yes, I am using a Thunderbolt cable. The cable is still powering the laptop just fine. I thought about taking it in to the Genius Bar at nearest Apple store but they do not recognize the 5K display as an Apple supported product even though I bought it from Apple.