kangoo not starting (no warning)

hi looking for a bit of help. I have a 2003 kangoo sl19dci 70. Drove to work this morning with no problems or warnings at all then when I went into the van at lunchtime it wouldnt start. The heater plug lights are coming on as normal but it wont fire up at all. I noticed after turning it over a few times the immobiliser light that normally flashes is staying on constant when the ignition is turned on. Any help much appreciated