It is always possible that your topic or product is less popular than it was a while ago - it would be a good idea to check that out first. However, if other people or companies with similar site content haven't lost traffic, it may be something to do with your site itself. You can use tools such as Google Analytics to check who is using it and when, but it can't really tell you why visitors are decreasing. The best thing you can do is try and get your visitors back up to former levels.
One thing you should do is increase your internet presence generally. If you don't make much use of Facebook and other media to link to your site, you definitely need to do that. You could also have a blog, separate from your site but linked to it.
For the site itself, check the key words and see which of these are no longer coming up in an internet search. You may need to rewrite some of the content using different key words. It can be a good idea to employ a writer with SEO experience to do this, as it's harder than it looks.
You can also find some very helpful ideas here.