HVAC unit won't come on
The first thing to check is if the air handler will come on when you turn the heat on. If the air handler will kick on and circulate air but not when you call for AC than you may have low refrigerant pressure.

Most AC systems have a switch that prevents the AC from running with low pressure to avoid damage to the compressor. If this is the case then I'd recommend calling a local AC repair service to have them charge the system.