Cannot push to Heroku 'fatal: unable to access..Could not resolve host: (nil); nodename nor servname provided, or not known'
Whenever this randomly happens to me, removing and adding heroku again as a remote reference always works.
First check if you do have heroku as a remote.
git remote -v
If heroku is present, remove it.
git remote rm heroku
Then add it back.
git remote add heroku git@heroku.com:project.git
Usually this works with me, try it and let me know what happens.