The answer to this question used to be simple. Now, with all the electronic components integrated in the modern cars, it could be a hard, if not impossible, answer to give without connecting the car to a computer.
There are though some tricks you can try if you knew your way around an engine. The most important thing to check is that to the engine goes the fuel flux. First of all check if your fuel indicator is not broken and you still have fuel in the tank. After that try to identify the fuel lines and find a method to see if the fuel pump works. This can be more difficult than it seems. For diesel engines it is not recommended to remove fuel lines because for most cars the system is pressurized. For gasoline engines you can remove a fuel line to see if there was fuel or the pump works. The lack of fuel can be also caused by a safety feature that every car incorporates. There is a system that cuts the fuel if the car had an accident. Also this feature can be deployed if you drove over hard roads or over really big bumps. So, the solution is to find the button that cuts the fuel, and press it. Usually, the button is hidden somewhere under the dashboard, in the glove box area. Find out where is it for your car.
Another reason why the a car won’t start is the electrical system. Check the condition of the battery by hearing how fast is the starting motor turning. If it is at a slow pace, leave it; you don’t want to remain without current. The start of an internal combustion engine requires a lot of current and a car battery can do about 4 or 5 consecutive starts until it remains without power. Another electrical component to check are the sparkplugs for the gasoline engine. This check can be a bit tricky so it’s not recommended to be executed by anyone. If you wanted to know how it’s done on the side of the road, without instruments, ask at an automotive service. Diesel engine have also some spark plugs but they work in a different manner. They only heat themselves in order to heat the air inside the cylinder to make the starting process easier. They break down relatively hard and this can cause problems mainly in the cold season.
If any of this methods doesn’t work, call a specialist either to repair the car there either to pick the car on a platform.