You should first check to see if you are getting 120VAC to the unit. Remove the inside cover and you will see a junction box. First, with the breaker off (unless you want us to start calling you 'Sparkie'
), remove the cover and untwist the white and black wire nuts. Keeping them separate, turn the breaker on and see if you are getting voltage at this point. If yes, you have a problem with the unit.....if no, you have a problem at the breaker box OR the wires that are coming FROM the breaker box to the A/C unit. Also, make sure that all of the connections in the J-box are tight if voltage is present. You might have a bad breaker even though it seems to work.
To check this, remove the hot wire from the breaker and the breaker from the panel. Do a continuity test on the breaker to see if you are getting anything across it. If not....replace it and that should solve your problem. If you have voltage at the A/C junction box, we will have to look deeper.
You also might have fried the start capacitor and then possibly the run capacitor in the upper unit. They are located in the upper shroud next to each other so you have to get on the roof to remove that. They will look wierd, fried or bulging most of the time if they are bad. I'm not sure of what unit you have, but if you have a PTCR, that might be gone also. If you can read the micro farad spec on the capacitors, you have to disconnect all of the wires to check the readings, provided that your meter reads capacitance. You should also be certain that the caps are discharged FIRST!!! BTW, AC power should be OFF for this test!!!
Keep us posted on what you find.