Basically there are many reasons of traffic declining, some of them are:
- Reduce in keyword ranking.
- Your content may not be good enough to engage users
- May be you lost some of the effective back-links for your site.
- Last but not the least may be your website or some important landing pages got a hit from Google.
These are some of the known issues of reduced traffic, but there is a possibility that these are not true, to get more information, it is better that you should share your SEO practices that you are following, so that we can share the possible reason for your situation and if possible the best solution to overcome it.