Here at Moodle Towers we get the occasional frantic email, as follows:
My course has disappeared! I can see the top section and this week, but the rest has all been deleted! Waaaaaaaaaaaaaaaah etc.
Don't panic! What has actually happened is that Moodle has gone into 'focused' view, where it shows just one section (other than the top one).
You can change back to normal view by clicking the button, over on the right side of the section.
Alternatively, just stay in focused view, but use the menu at the bottom to jump to other sections as needed.