Q:

git error "Please tell me who you are." and Heroku

I was working in branch master, and committing to Git repository. Everything worked fine. I connected new app to this repository on Heroku. 
I was committing to both Heroku and Git. Everything worked fine again (except I cannot run db:migrate on Heroku but that is another question...). After my last commit I run git status and received: On branch master. Your branch is up-to-date with 'origin/master'. 
Today I made some changes in my code, but suddenly I cannot commit - received error "Please tell me who you are." The only difference from previous commits is that had a migration, if it is of any importance. 
When I run git config --global --get user.emailcommand I get an empty line in return.
Why have I suddenly lost connection to git? 

A:

It seems that your email, and name parameters in global config are empty. Probably you have executed something like this, that has dropped the values:
git config --global user.name ""
git config --global user.email ""

Just fill them with commands again:
git config --global user.name "Your Name"
git config --global user.email "your@email.com"

Where your@email.com is your github email.
NOTE: You can push out of hand to heroku, bypassing the github, since heroku also have git repo itself.

