Customer described Furnace working intermittently, then not working at all. Upon arrival furnace not operating. Found high voltage incoming, but no low voltage throughout the system. Found loose neutral connection at the transformer, tightened spade connector reconnected to transformer and furnace started up. Checked unit for other issues. Found a clogged, dirty air filter. Removed it from the furnace. Explained to customer maintenance procedures on filter replacement and reccomend our annual maintenance plan to verify operations on furnace.