why won't my hps light work?

it's a 7o watt hps light bulb and i have a 75 watt lamp clamp and it won't turn on why?

not sure about this, but would you need a ballast?

probably not it's only a 70, i don't know tough.

all hid lights need a ballast. IF you bought that bulb take it back it's useless to you.

Click this link twice for a cheap hps that includes a ballast:
http://www.e-conolight.com/Product/EProductDetail.asp?ProductFamilyID=7&FGNumber=E-MT6H151G

You need to get something like that, a whole fixture. Sometimes they are on clearance at home depot etcbut you're not likely to ever beat that deal above.