MH help

i have a 400 watt mh light which i used for three months in veg i was going to flower with it but one week into flowering it stopped working so i went out and bought a 400 watt hps light but i read that having both lights in the flower room will increase potency i want to see if this is true by where the lights overlap and see how much better the buds are.
anywayz when u plug in the mh light it powers up and makes a humming noise like it always did but now the light wont switch on does any one know what the problem is and how i could fix it or is there something i can do to see if the wires are still good the light is probably 6 years old but the bulb is brand new