Citroen Jumpy does not start

Hello, I have a Citroen Jumpy. The other day I was driving along peacefully and stopped at a traffic light when it suddenly shut off and hasn't restarted since. I took it to the official dealership and it's been there for a week and they say they don't know what the problem is. I have some mechanical knowledge, but I don't know if anyone can guide me on what the problem might be. It hasn't shown any symptoms before, no unusual noises, smoke, or overheating. If anyone could help me I would be very grateful, as I use it for work and I urgently need it... Thank you very much