Will not turn on. Worked fine three months ago...

sounds like you have a bad transformer , if this unit is ran by a t-stat then take the wires of the stat and twist red,yell.,green together if unit comes on your t-stat is bad if not your transformer is bad cost to fix apprx. 60.00 to 100.00 depending on where you take it if you fix your self transformer probly 30.00 tstat about same,hope this helps