The dimmer for a ceiling light/fan combo in the bedroom stopped working. When trying to replace it with an electronic dimmer swich, nothing would work, so I measured voltage from the wires in the box. I had 122V from black to black but 81V from red to black. What would cause such disparity?

Still not able to get the light or fan to work.

Light or fan switch being on (at pull chain). Not a proper voltage check. Proper voltage check is between hot an neutral or ground. If switch is on the light bulb is just like a resistor. You are measuring voltage between hot and neutral with a resistor in line.
Make a direct connection between hot and black and between hot and red to insure that every thing works and confirm that problem is switch.

Is this a double post? I answered this question 15 minutes ago but my answer isn't here? You have 2 black a red and no white or green? Where do you live. Are the 2 blacks both attached to the switch?

Thank you. Pulled chain and switch became live. As for wiring in box, it is a double with two wire sets coming in and the two switches being wired together with black wires, all white wires attached together but not connected to the switches, the dimmer was connected to the red and black wires and the ground had been removed. Installed new switch with its ground wire intact.