A furnace switch can get bumped or turned off while changing a filter.  If you are not sure if there is power to the furnace, try turning the fan switch on the thermostat from auto to on.  The furnace blower should come on if there is power to the furnace and the blower motor and fan relay are working.

 

A breaker may be turned off inadvertently or may trip because of a power surge. Many times it may not appear to be tripped. Resetting the breaker by turning it off, then back on may solve the problem.