MH 150W Fixture

I have a large outdoor fixture that is about 20 years old. The light went out and I tried a new bulb that did not work, so assumed it was the ballast. The light was in very good shape, so thought I would try to fix it. The ballast was an Advance brand. The label on the old ballast had disintegrated, so all I had was a stamped number on the side. I called Advance directly, uploaded some pics to them. The Advance rep said it was too old to identify, but recommended a ballast that should work, so I ordered it. The ballast I received was at least half the size of the old one! I called back and reported and he said just probable newer technology hence a smaller size. So I rebuilt the fixture, hooked a plug up and it worked fine. I installed the fixture, which has a photo cell on it a ways from the light on a exterior junction box. I tested it and it worked. A few days later I noticed it quit working? The bulb was brand new, but didn't know how to test it. What would be my best next step?