One of the most common reasons a headlamp fuse blows is a failed bulb. It doesn't always happen when a bulb fails, but the filament can sometimes break (or melt) in such a way that it shorts to earth and allows 20A+ of current to flow. Most of the time this will only happen for a slit second (enough to blow the fuse), but occasionally (more by chance than anything else) the dead bulb stays shorted and then there is no point replacing the fuse until the defective bulb is replaced.

Replace the bulb, then the fuse and you should be good to go. Don't try changing the fuse and then the bulb, or you'll just blow you fuse again as soon as the engine is started.

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