Over time it is usual for a website to start to build up broken links. These are usually caused by content being updated or removed and internal links left behind. Sometimes these links are not always easy to see as they may be round an image or within the main bulk of content. There are many programs or even websites you can use to check your website and find out if you have any. Usually it will tell you where the link is found. With this information you can then go in and start to update or remove the broken links. Webmaster Tools also allows you to check for broken links and will show you the number of times and where it is linked from.
Broken links can not only be very frustrating for your customers, but can also cause a drop in the search engine rankings. Often your site will not start to recover until all the broken links are fixed and even then it can take time for your site to move back up to where it may have been previously.
Some programs will even let you set up alerts so you can see when broken links have been found.