Unless it's over 20 years old, it's electronic. The coil unit is self-contained, and is considerably more leliable thatn the older points type (and will retro-fit onto older engines), but they DO fail.
Usually they just die completely and produce no spark, but they can also 'hot fail' - IE they work until the engine gets hot. Usually, this happens if the engine has overheated at some time.
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Phil Saunders, June 2009