Screen was grey with prohibitory symbol...I used command r which took me to OS X utilities screen.....what option do I choose

I am not computer literate. I am using MacBook (2012)... I realized that I needed to update operating system tonight. the laptop went to restart and screen turned grey. I was able to press command r and now the OS X utilities screen is up. I do not have an external backup

restore from time machine backup

reinstall osx

get help online

disk utility

anybody able to offer assistance?