Why is my jekyll command not working anymore?

I also don't work with Ruby, but I had a similar problem on Mac OS 10.12.6 and here is how I solved it.

This is likely caused by some disagreement between the system Ruby environment and Jekyll, and the easiest way to fix it is to create a new environment altogether.

First you should use rvm to manage your ruby versions to avoid delete the system Ruby and break your OS. After you set up rvm, run

rvm install 2.3.3

to install Ruby 2.3.3 (which is what I used). After it finished, make this the default Ruby version in your system:

rvm --default use 2.3.3

then I did the following:

gem install bundler
gem install jekyll
bundle install

and Jekyll worked for me again.

Hope that helps.