As far as I know the Hdi and the 1.9 are both 'Interference' engines - especially with the high compression ratio needed to run a diesel engine.
This means that there isn't sufficient clearance between the fully open valve and the top of the piston at TDC.
So a snapped cambelt is usually an expensive repair. Sometimes they just need new rocker arms and the valves are fine.
What I find of note is that you were just travelling along in 5th gear and the engine died. You made no mention of a big bang and clatter that went with it. Hence the reason why Neil suggested the anti theft device not allowing fuel through to the pump.
If I were you I would be down to that garage first thing in the morning and inspect the damage.
Citroen recommend a cambelt change every 80k or 5 years, but most in the trade now feel that 100k is ok. While you will get the problem ones, I wouldn't take it for granted that it was a cambelt issue IMHO.
We have an 03 plate Hdi Xsara 110 within 101k on the clock - cambelt not changed yet, car purchased with 69k on the clock four years ago. My son's 03 plate Xsara was replaced last year on 129k. The old belt and idler pulleys were fine - I inspected them. The idler bearings ran smoothly although they were worn.
Spruce