I have a digital t-stat and it went blank so I have no a/c. I ohmed all the wires leading to the t-stat and they checked out OK. I checked the secondary side of the transformer and it checks out. I changed the t-stat and it still doesn't work. There is a diode between some wires but I'm not sure if that could be the problem. Please give me something else to look for.

Some digital thermostats have small batteries in them for the program, lose battery can lose the program, or all functionality of the stat.