My 2005 G35 Infiniti Won't Start!
 

One day I left my car on, listening to the radio and eating lunch. Then I notice my dashboard lights flickering as if my car is losing power. So I quickly turned off the car. When I tried to start it back up, dead, the car didn't even crank. I got a friend to give me a jumpstart since at that moment I'm thinking the battery died. Got the battery charged up a bit, the car cranks but won't start! Tried several times, either the battery ends up dying again or the car cranks and won't start. Got it towed to a mechanic shop, got a brand new battery, but the same thing, cranks but won't start. Any idea what's going on?

Thats really all your mechanic did was replace the battery? No further tests? No codes?
Have him test the voltage on the starter. Check all the grounds. Sounds like a power supply issue

Could be many things, do you have fuel in the tank? The gas gauge is notorious for failing so try adding 1gal fuel and crank it again. If it still won't fire remove the fuel pump relay and crank the motor a few more times to relieve pressure. Remove one spark plug, plug it back into the coil pack, hold it about 1/4" away from the intake manifold (wear leather gloves) while someone else cranks the motor and see if you have spark.

If you DO have spark then it might be a failing fuel pump, order the sandwich adapter for the fuel damper from Z1 Motorsports so you can use a fuel pressure gauge to verify proper pressure.

If you DONT have spark (and no solid red light on the dashboard NATS indicator) then it's probably your crank position sensor.

If you DO have a solid red light on the NATS indicator on the dash then it's probably your key transponder died or your BCM went out.

Of course
 

Yeah there were multiple tests, but they have to find the problem manually because they were not able to find an issue with the code reader.

That's also why if the vehicle does need to go to a mechanic it's typically cheaper and easier just to take it to the dealership for a diagnostic. $125 for an hour at the dealership who has Consult III which tests and checks ALL aspects of the vehicle is better then a shop that doesn't have the right tools for the job.

After Nissan tells you what the issue is you can decide how to proceed with repairs be it the dealership, private mechanic, or fix it yourself.