I am working on a lennox ac unit, when I first looked at it everything was running but the blower fan on the inside was not running. I changed the thermostat and now nothing is working . I think it might be the circuit board. Anyone have any ideas?

when I first looked at it everything was running but the blower fan on the inside was not running.

Why would you change a thermostat when the blower was the problem?
Put the old thermostat back on just the way it was then post back.

Never compound a problem by possibly causing another problem!!

Yes, please do as HVAC1000 says, and let us know what you find. If old thermostat also makes nothing come on now, then you probably blew a circuit board fuse(usually 3 amp). I'm betting that your original problem is the blower motor or capacitor, but now you've made it a bigger problem. No worrys, not much bigger of a problem. Just keep in touch and we will try to help you through this. Good luck. Lee.

PS. If this is a conventional unit, turn power off to furnace, remover thermostat, then jump r,y,and g at the furnace control board, then turn power on to furnace. This should cause outside unit and inside blower to come on. If not, let us know what does come on. This will tell you if your problem was in the thremostat(most likely not) or if problems is elseware(most likely motor).