Give us more info on the fixture, dual lamp, more? Many dual lamp configurations running on a single ballast will do this if one of the lamps is weak or on the way out. It's the EOL or 'end of life' safety feature of many of the newer T5 ballasts. The ballast will sense one of the lamps is at the end of its life and turn both off after a couple of seconds.
Try a new set of lamps before you get too concerned over a possible problem with the ballast. It's an easy way to narrow down the problem.
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