Exercise 2: A Footer Menu Disappears After a Theme Switch — Possible Solution
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WHAT MOST LIKELY HAPPENED
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The new theme most likely doesn't define a "Footer Menu" location the
same way the previous theme did, so the existing menu is no longer
assigned to any location the new theme actually displays.

WHY, PER THIS CHAPTER'S OWN MATERIAL
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Per this chapter's own warn-box, "where a menu can actually appear...
is a set of locations the active theme itself defines," and "switching
to a different theme can mean reassigning your menus to that theme's
own, differently-named locations." The menu itself still exists (which
matches the scenario, since it's confirmed still present under
Appearance -> Menus) - what's missing is the ASSIGNMENT of that menu to
a specific display location the new theme recognizes.

WHY THE MENU CONTENT SURVIVING BUT THE DISPLAY BREAKING MAKES SENSE
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A menu (the list of links and their structure) and a menu LOCATION (a
theme-defined slot where a menu can be displayed) are two separate
things. Switching themes doesn't delete or modify the menu's own
content, since that data isn't tied to the theme - but the new theme
likely names its own footer-area location differently (or doesn't
define a footer menu location at all), so the previously-assigned menu
has nothing left mapped to it to display in.

WHAT THE FIX ACTUALLY IS
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The site owner needs to go back to Appearance -> Menus, and re-assign
the existing (still-intact) footer menu to whichever location the new
theme actually provides for it - which may have a different name, or
may require checking the new theme's own documentation to find the
correct equivalent location, if one exists at all.

WHY THIS WORKS AS AN ANSWER
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It identifies the specific mechanism this chapter names (theme-defined
menu locations) as the cause, distinguishes a menu's own content from
its location assignment to explain why the menu wasn't deleted but its
display still broke, and names the concrete fix consistent with this
chapter's own explanation.