Every detail you provide not only speeds things up for you but at the same time keeps this forum topic from becoming a bit chatty and drawn out
The problem will be somewhere outside, most likely. The LV wires to the condenser are often cut by trimmers, chewed by dogs and rodents, etc. Go out there and carefully examine the wires to the unit (LV) all the way into the control panel to the contactor. A single loose connection under a wire nut somewhere can ruin your day. Check the contactor for signs of fire ant infestation and/or burned contacts while you're at it.
You can also attach your voltmeter leads across the 2 LV wires out there and go inside to set the thermostat to "cool". The reading should be 24 0r 25 volts to the wires at the unit. If voltage is present, replace the contactor. If no voltage is there, the thermostat may be at fault. Pay attention to the new thermostat wiring instructions when you get ready to replace it. Sometimes they need a jumper here or there, and they are all different in setup.