One of your headlamp bulbs has blown and you're probably now asking "What headlight bulb do I need?" shortly followed by "Where can I get the headlight bulb replaced?".
You'll find the first answer in your Haynes manual, while the answer to the second question is "right outside your home", because this is a job you can do yourself. On most cars you can access the rear of the headlight unit (which is where the bulbs are removed and replaced) with it in situ. On other cars you may need to remove the headlight, which sounds daunting but is in fact fairly straightforward. On some cars you may need to remove even more parts to get at the headlight, but your Haynes manual will show you how to do that, in a step-by-step format.
First off, finding out what sort of bulb it is that's blown. Your car's handbook will prove really useful here, as will this guide to the different types of bulb used in car headlights.
If your headlights cast a very bright, white beam at night, there's a good chance your bulbs are HID or high-intensity discharge, and are filled with xenon gas. You can also change these yourself, although the process is slightly more complicated because you'll need to disconnect your battery. Again, your Haynes manual will tell you how to do that. Here's an explanation of the differences between HID and the more conventional halogen bulbs.
How much does a headlight bulb cost and what sort of tools might you need to change a headlamp bulb? You'll find that information as part of our Home Mechanic Guides.
Ever wondered how long a headlight bulb should last for? OEM (original equipment manufacturer) bulbs – the ones the car comes with from the factory – should last longer than replacement bulbs you'll buy in a car accessories shop, while high-performance bulbs that are designed to produce a stronger (but still road-legal) beam of light tend to have a much shorter life. Find out more on that here.