What happened? It left the mothership (ugh) Mars Express Orbiter in a nominal manner, but then probably crashed, probably probably because the height detection part of the freshly-developed and innovative landing system (aero braking, chutes, airbags) sent false data.
A comission of enquiry has critised the handling of risk in the project.
Why? Start here.
24/5/04: "Report won't be published, only ... recommendations" Press Conf today ...
24/5/04: So, space robots are tricky, but surely the answer isn't space men? Link