the damage will be extensive if you have run it, petrol engines don't like deisel one bit! you can get away with it the other way round. but the whole engine will need cleaning out .
Depends how much you've put in the tank, but it will normally need the tank completely draining and new fuel filter fitting (depending on car model) new clean fuel and hopefully it should be ok, might take a few attempts to start but shouldn't be any long term damage
Depends on the car of course but with my old Escort I did exactly that and didn't notice till I got 1/2 mile on and the thing was coughing and spluttering away.
I didn't quite manage to get home so had the Mrs tow me the rest.
So I put the wifes diesel car next to the escort and bought a cheap washer motor from Halfords. A bit of hose from one tank to the other and it emptied the tank no problem. Went and got 2 gal of unleaded, put it in and within 2 minutes mine was back running as good as new.
One of the car TV prog's did the same with similar results.