Firstly, don't panic! You can always re-add a section block.
First, hover underneath any element on the right side of the page. This will display the blue Add New Element line.
Next, click on the blue line to add a new element.
From the Add New Element library, select the Section element.
There you have it, the section block has been added back to your page!
📌 Pro Tip: If you had accidentally deleted a section that you want to try to get back, you can try clicking the reload icon in your browser window. This will revert the page back to the way it was the last time you clicked the Save button.