How to get back Homebrew to previous state?

Question

I deleted some of .rb files(cassandra.rb,cassandra@2.1.rb and cassandra@2.2.rb) from folder /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formul

Answer 1

Try this command to bring back all of deleted files.
Note: This command will revert all change in $(brew --repo homebrew/core)

cd $(brew --repo homebrew/core) && git reset --hard origin/master

Homebrew uses git internally, so use git commands to repair some problem.

Answer 2

Install directly from GitHub as mentioned here - How to restore brew formula after I rm it

brew install https://github.com/Homebrew/homebrew-core/tree/master/Formula/cassandra.rb