Jekyll not serving locally - 'could not find jekyll' - ruby

Question

I have little-to-no experience with ruby, but have been managing to successfully maintain a Jekyll site with github. However, for some unknown reason, when I cd into my website's directory and run jekyll serve I get the following error:
Could not find 'jekyll' (>= 0) among 7 total gem(s) (Gem::MissingSpecError)
Here is the full error message:
/usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/rubygems/dependency.rb:308:in `to_specs': Could not find 'jekyll' (>= 0) among 7 total gem(s) (Gem::MissingSpecError)
Checked in 'GEM_PATH=/Users/amin/.gem/ruby/2.4.0:/usr/local/lib/ruby/gems/2.4.0:/usr/local/Cellar/ruby/2.4.1_1/lib/ruby/gems/2.4.0', execute `gem env` for more information
from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/rubygems/dependency.rb:320:in `to_spec'
from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/rubygems/core_ext/kernel_gem.rb:65:in `gem'
from /usr/local/bin/jekyll:22:in `<main>'
Trying gem env gives this, which looks a little messy:
RubyGems Environment:
- RUBYGEMS VERSION: 2.4.5
- RUBY VERSION: 2.2.2 (2015-04-13 patchlevel 95) [x86_64-darwin15]
- INSTALLATION DIRECTORY: /usr/local/lib/ruby/gems/2.2.0
- RUBY EXECUTABLE: /usr/local/opt/ruby/bin/ruby
- EXECUTABLE DIRECTORY: /usr/local/bin
- SPEC CACHE DIRECTORY: /Users/amin/.gem/specs
- SYSTEM CONFIGURATION DIRECTORY: /usr/local/Cellar/ruby/2.2.2/etc
- RUBYGEMS PLATFORMS:
- ruby
- x86_64-darwin-15
- GEM PATHS:
- /usr/local/lib/ruby/gems/2.2.0
- /Users/amin/.gem/ruby/2.2.0
- /usr/local/Cellar/ruby/2.2.2/lib/ruby/gems/2.2.0
- GEM CONFIGURATION:
- :update_sources => true
- :verbose => true
- :backtrace => false
- :bulk_threshold => 1000
- REMOTE SOURCES:
- https://rubygems.org/
- SHELL PATH:
- /Users/amin/Dropbox/code/google-cloud-platform/google-cloud-sdk/bin
- //anaconda/bin
- /opt/local/bin
- /opt/local/sbin
- /opt/local/bin
- /opt/local/sbin
- /Library/Frameworks/Python.framework/Versions/3.4/bin
- /usr/local/bin
- /usr/bin
- /bin
- /usr/sbin
- /sbin
- /opt/X11/bin
- /Library/TeX/texbin
My best guess is that Jekyll should be looking in usr/local/bin, and not usr/local/Cellar, but as I say, that's just a guess.
Any advice on how to get this working again would be greatly appreciated!