HPS advice needed (bulb life and technical difficulties!!)

Ok so I bought a 400watt HPS, had it on for 18hours a day for the last couple months and it was fine yesterday night when i turned it off, this morning when i turn ot on at the wall the ballast buzzes for a few seconds then nothing. I leave it switched on and every few minutes it buzzes again for afew second, but no light !!!!]
has the bulb run its course and ran out of juice or is it a problem with something else ?? advice very much appreciated, expected the bulb to last far far longer than a couple months, dont want to buy a replacement incase its the ballast or light fixture thats broken??
please help !!!

If the bulb is fine, not popped=you will see it.Then most likely the ballast.What kind of ballast you have?

Have the bulb checked at a local shop, if you can't I would replace the bulb . It's cheaper to replace & you will need one eventually anyway. Bulbs burn out far more often than ballasts.