What are things that we need to check in priority when site traffic goes down suddenly? I just want to know major factor which can down site traffic
There are a number of possibilities.
First thing you should do is compare your Google Analytics with any algorithm updates. If there has been an update around the same time your traffic tanked, then that is likely the problem.
I would also check webmaster tools for any messages and manual penalties. This will entail a lot of work cleaning up your inbound link profile.
Another possibility could be a website error. Make sure robots.txt is not blocking things that it shouldn't, make sure that there are no robots "noindex,follow" meta tags. The former is easier to restore, I have noticed pages that have taken extreme amounts of effort to recover from the latter.
If applicable, please check your .htaccess (or equivalent) too. Errors here can cause havoc. It is possible to instruct search engines not to index pages in the .htaccess.
If you give more details, such as dates and google analytics screenshots with the dip, I can provide a more accurate response.