Transmission "hold"
 

I have a 2000 Millenium S with about ~160K miles. This morning, I went to plug in my phone charger, and knocked the hold button on the shift lever. I pressed the button again, and the hold light did not go out. Now the car will not shift into overdrive. The hold light is not blinking, it's on steady. the CEL is not on and there are no codes coming up on my scanner. The car appears to be running fine, otherwise. I am thinking it is either a stuck solenoid, or a bad switch. Any thoughts?

The switch only provides a momentary ground. It is probably the switch. Try disconnecting the battery (neg side) and stepping on the brake pedal to drain the PCM memory.

Tried your suggestion this morning.....no joy. Are you saying that the switch is stuck in the closed position? How can I check it?

The switch in the shifter sends a signal to the TCM, the TCM kicks the solenoid in the valve body ( simple on/off solenoid) I would disconnect the mode switch on the shifter and with an OHM meter check it. It has two terminals, with the OHM meter across those terminals work the switch. It should have continuity and then no continuity when pressed again. That model changed. I stated that it was a momentary ground circuit but they changed it. It should have 12 volts on one pin going into the switch. If not, check fuses. If it needs a switch then you must replace the shifter handle assembly to get the switch.