If you change your filter and now your system doesn't work check the door on the furnace it may not be on right. There is a switch on the door that turn off the system. You might want to check the power switch many furnaces have a switch next to the light and you might have turned it off by mistake. Some furnaces have safety switches on the pan you may have moved it while changing the filter. We found these problems before they are common. If your air conditioner still doesn't work call for repair.