how can i fix my ldv pilot van 1.9 reg t324pda?

my ldv pilot van has died suddenly as i parked up and turned ign off the battery light came on i tried to re start and got nothing at all i have tried starter motor toook it off and tests ok have also put new 12v relay on am at a total loss and unable to go to work itis a 1999 ldv 1.9 pilot van diesel