The screen on the programmable thermostat went blank when I switched from heat to cool with the mode button. I have 27 volts dc out of the transformer and only 13.5 volts dc at the thermostat. The led light blinks two times and then three times and then repeats itself. I have checked the pressure switch and it is clear.

You should be reading AC volts, not DC. Measured between R & C you should get around 26 volts. Probably 28 or a bit over. If you do, reinstall the stat. If it is still blank, the stat has probably had it.
What does that particular blink code mean?