Story house, one 75gal water heater. Downstairs shower and all faucets, including upstairs faucets have hot water. Showered in the upstairs showers (bath/shower combo style) and initially I had a hot shower. Now, all of the sudden, no hot water whatsoever ever in either upstairs tub!! Faucets in same bathroom have instantaneous hot water... Tubs, nothing. Left it in for 5 min. No change. It's a single handle style and let it run in all areas of the "clock" per se. How is this happening ???