Why doesn't my ac unit turn on?

There are many reasons why it does not work: its temperature control may be set too high if you want cooling or it may be set too low if you want heating; it may have lost all its refrigerant because of a leak in the system; its compressor pump may have stopped working; etc...

If you cannot see anything obviously "set wrong" the best advice anyone should give you is to call a licensed Heating, Ventilation and Air Conditioning (HVAC) technician to advise you and/or fix it for you.