Why doesn't my central air does not come on?

This question "Why doesn't my central air does not come on" is hard to understand. What does it mean? "Why doesn't my central air come on" may be a bit easier to understand but it may still be wrong. It is not clear what is meant by "air"... Is the question perhaps asking about an air vent in a car? Sometimes a leaf or something else can get sucked in to a car's fresh air system.

A car's electric fan can sometimes break a leaf into small pieces. The small pieces then get blown into the car through the air vents.

But it can happen that the pieces are too big to get through. They build up a blockage behind the inside of the grille of the air vent.

When that happens you can usually see the blockage behind the grille.

It is usually possible to clear the pieces by poking at them using something like a long thin screwdriver or knitting pin! Hope this helps but if you meant to ask about something else - not a car's air vent - then please ask another question to say exactly what it is that does not come on.