Have a criterion gas furnace... was working, now fan doesn't come on... even when controller is put in "fan only" mode.. ideas?

Would check thermostat make sure nothing touching green wire and or make sure green wire is connect use meter to make sure 24 v is getting to it at therm stat, one lead on green one on red do same reading on board be very careful test green wire where comes off the board out and up to thermostat ,
If have 24v,, both places then would start checking Volt and or current to motor if getting correct reading everywhere have to check your safey switchs, make sure wires in right place on board black to cool might want to check you scamatics for that step every motor can have 3 to 5 wires mines has 5 speed wires go different area 2 are parked in not used areas, if nervous about testing then don't do it, if don't have training I wouldn't mess with it, could cost more than paying a repair man, may cost life,, at very least find someone that has taken at least basic electric