Start by checking the oil:
First use the dipstick to check that there's actually some oil in it. (NB: Remember to wipe the dipstick, and then reinsert it, rather than just looking at the end of it as soon as you extract it). If there's no oil, then there's a very good chance that the engine has seized. If so, you'll either need a new engine or a new car.
Next, assuming that there's actually some oil in the engine, remove the filler cap and peer inside. (A torch can be useful here). You're hoping to see the surface of the oil looking just like it would if you peered inside a container of oil that you'd just bought from a shop. If, instead, it looks like thick mayonnaise, then a head gasket has blown. (Probably around £800 to fix at an independent garage, or well over a grand at a main dealer. Alternatively consider getting a reconditioned engine fitted - or scrapping the car!).
Let's hope that neither of those problems turn out to be the fault though!