how to fix my 99 reg vauxhall omega automatic?

Is the engine spinning when you turn the key?
Is there a flashing light on the dash when you try to start the engine? If so it's probably an immobiliser fault.
If you have dropped the car key you may have dislodged or lost the 'chip' inside. Try using the spare key.
Another possibility is the gear selector switch failing and not recognising the 'park' position. The car wont start unless it's in park.

tug
April 2009