Before calling a HVAC company here is the simple hvac troubleshooting list of things you can perform yourself.
Make sure that the swith is on the correct position "HEAT/COLD". Is display showing anything. Make sure that the thermostat is set at proper room termerature.
If thermostat is digital it has batteries, are the batteries charged. If your thermostat has a switch to control the furnace fan, switch must be turned "ON". You should hear the fan working and the air should be coming up in the supply registers. If fan is not working there could be no power to the furnace.
Most of the systems are equipped with the furnace emergency "shut-off" switch. Make sure the switch is "ON". The switch is usually located on the side of the furnace and looks like regular switch. Sometimes the switch located near the furnace and possible was turned-off by accident. Sometimes there is an emergency cut-off switches that are activated when a door or service panel is removed, for example after filter cleaning. If the switch is on but still there is no air coming out you need to check your electrical panel.
Electrical Panel Troubleshooting
Find the electrical fuses for furnace & air conditioner on your home's main electrical panel and make sure that the fuse is now blown and if it is a breaker that is not turned off. Try to reset the breaker anyways by turning the breaker "ON" and "OFF". If the fuse is blown replace it with the same size fuze, its very important. If after all the breaker keeps breaking and if after installing the new fuse it keeps blowing contact your HVAC contractor.
You might also refer to the instruction troubleshooting guides that came with your furnaces and air conditioner, but only perform what is advised in the manual. If everything that you have tried failed call you local HVAC technician.
Don't try to fix anything yourself. All the repairs must be performed by licensed HVAC technician who went through extensive training and hold valid HVAC license.