Citroen Xantia car wont start

Ive never had this problem before. I ook the car out and it started as always first time. I switched of the engine while wife nipped into the shops, When I tried to start the car. I noticed that the only lights that came on were the Handbrake warning light and the battery light. As its a diesel .I normally have oil light ect, and I have to wait for the heater light to go out.I turned the key and the car turns over but would not start.I checked the fuses ect and they all seemed fine. sat in the car waiting for AA. after about an hour I turned the key and hey presto everything was as it should be. AA man could find no faults so drove home, put it in the garage,
Had to nip out in it and the same thing has happened again, Anyone any ideas Please