The plenum is frozen. No air is rising through the ducts. HELP.

Do you mean the evaporator is frozen? If so, You will need to bet a hair blower and defrost the ice. Then check it for cleanness, check your air filter. Is it clean? Is it installed properly? Is the condensing unit running? Is the indoor blower motor running?

I;m not sure about terminology. What is the evaporator? The condenser is running the blower motor is running, yet no cold air is rising through the ducts.

The evaporator is a coil inside the inside unit. It will be where the 2 copper pipes go into it. If it is frozen no air can flow through it, thus no air coming out of the vents