These kinds of problems are very difficult to diagnose without being on site. Rather than guessing at a single fix, let me give you a process to follow. Think of electricity as a closed water system, with hot water coming in and cold water draining out. What we are looking for is a leak in the system. I'd start at the breaker. Check the load screw to the ground/neutral buss. 120vac ok? yes, then we move further down the line to the next possible junction point. At each subsequent point, check your hot side to neutral and to ground until you find the point where there is no longer a proper 120vac measurement. When you find this point, if you have 120 hot to ground, but not to the neutral, you have an open neutral. Turn OFF the power and check the neutral to ground with an ohm meter. It should be very close to 0 ohms. If you see no voltage to either neutral or ground, and the neutral to ground is 0 ohms, then you have an open hot. Check the input side of the GFIC and the load side in a similar method. If you try to follow a very straight logic, testing the three possibilites at each point,(open ground/open neutral/open hot) hopefully you will find the culprit. Good Luck
Disconnect the power
Start by turning off the breaker, and pulling the serviceman disconnect, which will typically look something like this.
This will insure no electricity is flowing to the condenser unit while you're working.
Open the unit
Next you'll want to disassemble the unit, to allow access to the electrical parts. This will vary from unit to unit, so check the owners manual for the procedure for your unit. Once you have the unit opened up, make sure to discharge the capacitors.
These things store enough power to kill you, so you don't want them to discharge accidentally.
Resistance is not futile
Once the power is completely removed from the unit, it's safe to start poking around (electrically speaking, don't go busting the refrigerant lines). Start by tracing the wires from the condenser fan motor, back to where they connect in the electrical box. There should be 3 or 4 wires. In my unit, I had Black, White, Brown, and Brown with a White stripe (your model may vary). To determine if the motor is good, you'll measure the resistance across each coil. To do this, you'll have to disconnect the wires, so the motor is no longer part of the circuit (make note of where the wires connected).
Typically you'll have 3 wires, start, run, and common (we'll ignore my 4th wire in this answer). Set your multimeter to measure Ohms, and start measuring. You're going to measure the resistance between each combination of two wires to determine what each wire is, and if the motor is still good. Let's start with Black and White...
Black -> White = 15.9
Black -> Brown = 35.4
Brown -> White = 51.2
Knowing that...
Common -> Run = Lowest resistance
Common -> Start = Medium resistance
Start -> Run = Highest resistance
We can determine that...
Black = Common
White = Run
Brown = Start
If we also know that the two lower readings should always add up to the larger reading, we can safely say this motor is still good. If you measure 0 or infinity between any pair, that means you have a shorted or an open winding and the motor should be replaced.
Repeat the same procedure for the compressor motor.
Shorts on the ground
The other thing you'll want to check for, is shorts to ground. Set your multimeter up to test impedance. Put one probe on the equipment grounding conductor of the feeder, and the use the other to find a solid ground on the motor. You may have to scratch some of the paint off, especially on the compressor. Once you've found a solid ground, measure from each motor wire to your ground spot. If the meter beeps or give a low resistance reading, you have a short to ground. As with the resistance test above, the motor should be isolated from the circuit when doing this test (once a solid ground is located).
Best Answer
Simply put, you have a true short in the cabling someplace. To solve your problem: First of all be sure you are working with the affected outlets and fixtures and don't accidentally pull something out that is still powered up. Then you should pull out all the affected outlets and fixtures and look for any shorts. If none found, take pictures of each one BEFORE. Then disconnect everything including the connection to the breaker. Be sure that you aren't testing anything live by testing for voltage first. Then start testing with a DMM on the ohm setting. You should have near infinite resistance across the white and black in every case. If you find a short (very little resistance), you should be able to find the "other end" of the cable because there should be another pair with low resistance. Then, hooray, you found the cable with the short. What you do from there depends completely on where that cable runs, hopefully not to difficult a replacement. It would be good though, to identify the cause of the short. It's very unusual for the cable itself to be compromised.
But rodent damage can do it: I had a similar situation in my previous house. Some outlets and lights stopped working and another circuit started tripping breakers with nothing plugged into it. The affected outlets were on a wall shared with an attached garage. So I started pulling off sheetrock and this is what I found!