The process of setting up a GitHub account works just fine but it doesn\'t work when I try pushing my repository to GitHub. The error message it shows is as follows:
Github is currently (on 22nd of feb 2012, again on 5th of sept 2016) having some problems with pushing/pulling over ssh. I'm experiencing the same problems.
Pulling over https is working thought.
You can try pushing over https by following this guide: https://help.github.com/articles/using-ssh-over-the-https-port/
This would only help if https is not affected. you can try if https://status.github.com/ shows signs of things slowing down. Graphs where the mean response time goes up means there are issues (or possibly a DDOS) at github, and not at your end.
If github is currently being DDOS'ed they may choose to enable DDOS protection from a 3rd party. This would mean they change their dns records and/or servers if these are part of the DDOS.
Flushing your dns, changing your dns server or restarting your network (which will possibly flush your dns cache) as stated in the other answer might help in fixing the issue for you if github is mitigating a DDOS.
Or if you have problems with your ssh setup you can find the ssh faq here: http://help.github.com/ssh-issues/
Other ways to flush your dns, in windows in your terminal
ipconfig /flushdns
on macos
dscacheutil -flushcache
on linux
service nscd restart
or simply rebooting ;)