Expanding on what William Pursell said, you probably ended up in vim. Save your changes and exit the editor by typing : to enter a command, followed by wq, then press enter. To exit vim without saving your changes do :q! instead.
To change this default to something you’re more familiar with, you can set the EDITOR variable to something of your choice (try nano).
Just put export EDITOR=nano at the end of ~/.bash_profile (create the file if you don’t already have it) to get this behaviour for every new terminal session.
Also, you could do git commit --amend -m 'Your message here' without a need for an editor at all.