A section is gone from the menu

Two reasons, and from the sidebar they look identical: the item is simply not there, and if it was the only one in its group, the group is gone too. The tiles vanish from the control panel the same way.

It is switched off

The common one by a wide margin. Turn the module back on — see the "Module Catalog" guide, section "Turning a module on and off".

If its own page is open at that moment, a strip appears on it with a Where to turn it on button.

The settings are here and the module is not

Frequent right after settings are moved to another computer: a snapshot carries the list of modules and their settings, never the module files. Such a module has a record and nothing on disk, and the app keeps it switched off — which is why it hides exactly like the case above.

Two things tell them apart. In Module Catalog it is offered for installing instead of showing a switch. And its page, if it opens at all, says Module is not installed — "Its settings are here, but the module files are not. Install it in Module Catalog — the settings will be picked up."

Installing is the fix, not switching it on; the settings are already waiting and attach themselves. Doing it for the whole machine in one go is described in the "Backup" guide, section "Modules that were not installed".