It's just a cunning move on the part of the in-laws to saty at your house over Christmas Anders
"Sorry, we'll have to stay because the car won't start"
First quick checks will all be electical. Make sure the fuel pump is running. can you hear it whirr when you first turn on the ignition?
A total non-start will invariably be a loss of something electrical. Check all connectors are properly home.
Anders, I don't know why I'm telling you this

This is the kind of stuff you tell others every day
Is the immobiliser doing it's thing? If it's a keypad type, is the green light coming on OK. If it is remaining locked when cranking, it'll bleep.
If it's just very reluctant to start and runs very roughly, check the pipe leading to the MAP sensor if the sensor is mounted away from the inlet manifold. It perishes and/or blocks up.
Has it got petrol in it? Not the first time I've gone through all the checks just to find the tank is dry!!!!
Hope it sorts out Anders and in any case, a very happy Danish Christmas
