I have had a major tune-up done and the had the A/C fixed about 2 weeks ago and now my car won't start when it is very hot (more than 90 degrees) outside, come night time or when it rains, the car cranks right up as if nothing is wrong. .......I am so lost and confused......

 

could be the in tank fuel pump. If that suckers dead, when the tank is hot, the in line doesn't produce enough suction to get gas to the engine. However, cold gas is denser and thus easier to produce the suction with, i.e. cold gas tanks will start.