I Have had first hand experience with a few astras. My problem was sadly fatal as it was a Heat seized engine. Nothing can be done and its a new engine job, alot of the time this happens in the winter as alot of people put the wrong kind of oil in over summer. I hope it is not this for you.
- try to bump start the car
- try to jump the car off another battery / car.
Try knocking the started motor with a bit of wood, some times you get a lazy started motor which just does'nt wanna turn over. Try 3 or 4 times with the key then just give it a slight knock and try again.
Im hoping its just a one off lazy starter. Good luck !