Get yourself a cheap battery trickle charger and charge the battery overnight (remember to leave the acid cells open or they will explode). If in the morning you have a charge, it was a flat battery, if not, one or more of the cells is gone.
Since the vehicle runs fine once started, I doubt you have any problems with the alternator, although there could be some problem with the charging loom (unlikely)
Nico, July 2006