Website is unreachable because of Status Code:429 Too Many Requests. What should I do?

My site http://iosre.com is built upon discourse. It remains unreachable after setting up fail2ban. I've asked the question on their official forum, but their staff said it may be a general server problem and suggested superuser.

I guess my website may be under attack, but since I'm not a server expert, I have no idea what to do the fix or track down the problem.

Any ideas? Thanks!