Depends if it was the bulb or the ballast. a lot of the cheaper bulbs have no warranties while most cheap house brand bulbs have a 1 year warranty (ex: Maxlume,Ultra Sun)
If it was the ballast than It would say on the box or you could type your model. Most magnetic ballasts have a 5 year exchange warranty, while certain crappy like the budget grow II only come with a 2 year warranty. Most electronic ballasts have a 3 year plus 2 year prorated warranty. Your hydroponic supplier or your manufacturer are both liable to replace them if they are under warranty. I would start with who you bought it from