My engine was on for 10 straight hours with 8 hrs drive 2 hrs sitting on traffic. and suddenly A/C didn't work....no cold air...
After took 1 hr of break A/C started back..

so when the engine was cold few hrs later, I opened the hood to check coolant level, and found a white bottle behind radiator.

Coolant was almost topped off the radiator but the white bottle that was connected to radiator was empty....

does white bottle suppose to be full?

 

does the "bottle" get thin and wide/flat around the middle? if so, that's the coolant reservoir... and no, it's not supposed to be full. but there are two lines on flat area that marks low and max.

it's a sort of extra supply for when the radiator is low... and overflow if the radiator is full (so any extra coolant has a place to go).

if it's empty, but the radiator is full... maybe there's a clog in the system. need a coolant flush? or worse... taken apart.

 

The coolant overflow tank you are talking about has marks built into it for hot and cold and yes it needs to have coolant in it to the correct mark.

There are a bunch of threads on the A/C problem and what needs to be done to fix it look around the site a bit and you will find them.

 

I can not believe I'm answering AC questions in November.

The next time that happens. Turn of the AC but leave the fan on high for 5 minutes. Then turn on the AC again. If it then works, you have an evaporator freeze up problem. This happens when the evaporator runs too cold, accumulates moisture, and freezes the moisture blocking the cooling fins / coils. If this is your problem, a temporary help will be to run the AC on recirculate. It will get less fresh moisture laden air on recirculate.

The real problemis you are low on R-134 refrigerant. The system needs to be topped off or better yet the slow leak found, repaired, and then recharged.