When you create a new Microsoft Word document, you open a new document and can enter a lot of text, but until you save the file and give it a name, it doesn't exist as a permanent file.
When you create a new libguide, you immediately name it and save it. Every time you add a new page or a new box, you immediately save it. When you add a link in a list or a book from the catalog, you save it.
If you are adding or editing a lot of text within a box, periodically save it. Don't stop in the middle of adding a link to text within a box; if you edit other text within the box or save the text in the box before saving the link that you were in the middle of adding, you'll probably get an error.
As soon as you make a change in LibGuides, it happens. That means that if you delete a link, a box, a page, or an entire guide, it's gone. When you start to delete something, a warning will pop up asking if you're sure that you want to proceed. Don't just click through the pop ups without paying attention. Be sure that you selected the right item before you proceed or one of these days you'll probably delete a lot of work accidentally.
NOTE: The process of deleting an entire guide is simple...and ripe for accidentally selecting the wrong guide. From the list of guides that displays, you select the X in the right column to delete the guide. The lines listing the guides are close together and it is easy to accidentally select the line above or below the one intended. Make triply sure that you selected the correct guide.
In the Command Bar, the last link in the Guide Settings drop-down list is "Create Backup". You can save an html version of your guide which will have all of the text and links for all of the pages and subpages, but it will all be in one long page. You'll have the content of the guide, but if you have to reconstruct the entire guide, you'll need to rebuild the structure of tabs and subtabs and cross-links between pages.
If you have a guide with a really complex structure of tabs, subtabs, cross-links, etc. that would be a major project to reconstruct, consider copying the guide and renaming it with multiple z's at the beginning of the backup title so that it always lists at the bottom. Leave the backup copy completely unpublished.
NOTE: If you had to restore from this backup version, it's likely that you would need to edit the internal crosslinks, but that would be a relatively minor effort compared to recreating the entire guide.
The XML Export module provides some ability for us to periodically backup our entire system, but reloading pages from that backup can't be done by our institution's LibGuides system administrator; it would require providing the content to LibGuides and having them restore the page(s). If we had to request such restoration more than a time or two there might be a service charge, so just be careful and don't accidentally delete pages in the first place.