Furnace fires up just fine, and fan spins but there is absolutely no airflow in duct work. Any ideas?

Last spring thermostat went out and I replaced it, worked great. A month later circuit board burned out, tech found that problem and replaced it (major pocketbook ouch) and everything worked fine. Month or so later fan quit working no issue it was now summer. Unplugged fan gave it a separate power source and it worked. Plugged it back in and it now works. However there is absolutely no airflow in duct work.