if the ballast is humming,the god news is the most expensive part(the ballast) is most likely good.the HPS side needs a capacitor and an ignitor While the MH only uses a cap &normally, the cap has a different value between HPS &MH.pop the lid & check it out(here is where a picture of the inside would help). the ignitor will have 3 wires but the cap only 2.
since neither work, a good bet is the ignitor is ok & if the unit has 2 caps,the odds of both going are slim but if only 1 cap.....
my guess would be a faulty switch or poor connection.
WARNING!- the capacitor stores juice like a battery and can zap you even after being unplugged! carefully short the 2 terminals out to discharge before playing w/ any wires inside.