How can I make my 1997 Peugeot boxer (diesel) start again?

After running beautifully the Peugeot Boxer 1997 diesel van that we have is not starting.

We have filled the oil. The battery is fine and fully charged.

The ignition is turning but not igniting. What could the problem be?

We live in Sweden, and it is very cold at the moment, would this make a difference? (It hasn't before)

Any suggestions or advice much appreciated.