Back to square one
When you have to do something over again
It is a bad day, you finished your project and the wind blew it away. You're so furious and you've to do it all over again. This is what you called "Back to square one"
Back to square one
When you're back to where you started, all progress has been lost
Man, someone shredded my 500 page essay... well now I'm back to square one