If by "deleted" you mean you converted a black square to a white one, here's how to change it back:
1. Click on the square you want to change (the arrow cursor should now be in that square)
2. Choose Edit -> Square Properties from the menu
3. Click on Block in the Square Type area of the Square properties window
4. Click on OK in the bottom right
This will change the square to a block without changing any other squares in your grid.