1. Is the immobiliser active?Most modern cars are fitted with immobilisers which only allow the engine to start if the right key is inserted. Have a look at the dashboard while you turn the key and check whether the security warning light appears – it's usually shaped like a key, and it may flash. If you have a spare key, try this first. If that works, great! If not, you may need to get your key reprogrammed or get the dealer to provide a replacement.
2. Check the battery
This is an important thing to check since it's a very common problem. Maybe you just left the lights on overnight, or perhaps the battery itself is old and tired. It's possible that there's an electrical fault, too. Try turning on the windscreen wipers – if they're very slow, your battery level is low. Jump starting the vehicle is one way out, but be careful to follow the right procedure. Getting a new battery fitted is often the best option.