my profile Rank is showing NA any idea

Yes, it will get updated soon. Rank is a feature that is compared to all the users and then alloted to user on daily basis.

Yes it will keep on changing since your created recently it's happening don't worry keep contributing then it will not be NA

2 days back it was showing rank number but now it is showing NA    any idea?

A member ranking represents the contributions and the status of a member on the platform. C# Corner ranking is based on several criteria including contributions, quality of posts(articles, blog etc), shares, likes, comments, and several other parameters.