Go in the attic and trace the duct feeding that register. It may be disconnected or have a big hole in it.

Or you have a damper that has malfunctioned. Then again, you could be iced up if you have a heat pump this time of year.

Thanks for the logical ideas. I did check the ducts in the attic - as far as I could follow the one to the bedroom, all looked OK- no disconnects, no holes. It's crazy, I know, that's why I'm looking for ideas. Thanks again.

I assume this register is in the ceiling. Can you remove the register grille? Stick your hand inside to feel for any blockage.

If there is no blockage and your air handler is working properly then the air must be going somewhere else.

How well insulated is the duct? If not exceptionally well, it should feel warm if the heat has been running for a while. Compare the temp to the other ducts. If you want to get some quantitative information, IR, "point and shoot", thermometers have become really inexpensive. They make for great kitchen tools when you are not measuring duct temperatures.

Heat register is cold (room temp) to the touch. I reached my hand to my elbow, all clear. This is really nuts. Thank you for the thoughts.

I was thinking about the ducts in the attic leading to the register, are they warm to the touch?

Attic ducts are insulated, but do feel warmish.
Thanks.