To check if there is any problem/issue in Nintex site check it here : https://status.nintex.com/

For my part (In my tenant), I can Open workflow Designer without any problem. Check if you haven't any app like AdBlock or uBlock in your browser, they can prevent the opening of nintex Designer. 

@Abhishekn I'm also able to open the designer without issue. You might try seeing if this is effecting your sight with all browsers, as I've seen similar behavior before where it was a caching issue in Chrome or the like.

If you are still seeing the issue within all browsers, might be good to open a case with support as they can help you review some network logs that might identify the issue.

Thanks for your time. Now I am able to open the workflow. I open it in chrome. Previous I am opening in incognito.