rails s 時のエラー (Gem::GemNotFoundException)

Railsアプリ作成中に、rails sでサーバー起動しようとしたら、Gem::GemNotFoundException エラー。
とくになにかしたわけではないが、突如発生。
stack overflowで似たようなエラー事例があったので、参考にし、解決。
参考：stack overflow
rails s 時のエラー。

Traceback (most recent call last):
   4: from /home/ec2-user/.rvm/gems/ruby-2.6.0/bin/ruby_executable_hooks:24:in `<main>'
   3: from /home/ec2-user/.rvm/gems/ruby-2.6.0/bin/ruby_executable_hooks:24:in `eval'
   2: from /home/ec2-user/.rvm/gems/ruby-2.6.0/bin/rails:23:in `<main>'
   1: from /home/ec2-user/.rvm/rubies/ruby-2.6.0/lib/ruby/2.6.0/rubygems.rb:302:in `activate_bin_path'
/home/ec2-user/.rvm/rubies/ruby-2.6.0/lib/ruby/2.6.0/rubygems.rb:283:in `find_spec_for_exe': can't find gem railties (>= 0.a) with executable rails (Gem::GemNotFoundException)

以下で解決。

gem install bundler
bundle install