In my experience it's generally either the earth strap (as previously mentioned) or a failed cell in a battery. A failed cell or a dying battery (same thing really) can give enough juice for lights and things, but lacks the CCA (cranking amps or 'woof') to turn over the engine.
A booster pack is a handy and inexpensive thing to have knocking about, if you connect one and the car starts, then you know you've got a duff battery.
This is at the outer edge of my technical expertise.
