I concur with Walt. If it the thermostat is NOT calling for heat and the fan is running continuously you likely have a control board issue.
On my Seneca's Atwood furnace I can remove the 4 screws on the outside cover panel, and once the panel is off I can see the control board on the left-hand side of the furnace. It has a small red led on the board that flashes trouble codes when it senses a problem, yours might throw a code to help determine what is amiss.
Mine also has a on/off/reset switch outside under the panel. That switch also incorporates an internal circuit breaker according to my Atwood manual. Although I doubt turning that off then on would change anything, you might need to do that to trigger the control board into running another cycle to display any error codes if there are any. When that switch is off, my thermostats are dead, so they obviously somehow receive their 12-volts from the furnace circuitry. Good to know if the thermostats are dead but the power center fuse is good!
2014 Seneca 37TS
Toads: 2014 Ford Explorer Sport or 2013 Ford Focus ST