a/c not working

You have a short in the low voltage wiring OR a short in one of the devices that uses low voltage as a control factor.
First check outside to see if the low volt wiring going to the A/C unit is intact. Many times a dog or weed whip chews it up and causes a short.
The next items will need to be tested for a short or unhook one side of the low voltage control wiring to test. You can test buy OHM meter or by unhooking one low volt wire supplying each item ONE at a time then reactivating the unit to see if the transformer pops. LOL JUST kidding. Get a small automotive fuse holder and place inline between the low voltage output wire and the wire feeding from it. This way you will just pop a cheap fuse instead of the transformer every time you test.

I always prefer the OHM meter testing style since I do not have to power up the unit to test and I do not blow transformers or fuses. Good luck.