The 24v signal the board is telling you is the problem is where the thermostat gets its power from without the 24v to the thermostat nothing is going to work. Check your red wire in the furnace to be sure it didn't come loose or break. Also check to see if there is a 3 or 5 amp fuse in the board that is blown. If the fuse is blown there will be no 24v. If all these problems check out then the transformer could be bad this is where the furnace gets the 24v from.