HPS light cutting off?

Over the past 2 months my HPS light has gone out and I would turn off the balast and turn it back on and it would work.

What is making my light go out, but the bulb is not dead? I bought a new bulb today and tempted to put it in, but I don't know if it needs a new bulb..

Thanks

an hps bulb sometimes will still work if its bad. once it gets hot it turns off. then it will cool and restart.
put in the new bulb and watch to make sure it stays on. if it stays on for about 15 minutes you should be good.