WoW addons not working

Name the symptom first. A missing addon, a lost setting, and a post-patch failure are three different folders.

Original illustration of an addon settings panel

Original diagram of an addon folder window

Pick a symptom

Not in the list: wrong flavor, nested folder, or the character-screen checkbox. Start at not showing up.

Settings revert: they live in WTF, not in the addon folder. Start at not saving.

Worked yesterday: the out-of-date box reset, or Midnight hid the combat data. Start at after a patch.

A Lua error is a fourth door. The Questie project documents /console scriptErrors 1 to reveal the error. Read the addon name in that error before you delete the whole Interface folder.

Questions

Should I delete WTF first?

No. Rename Cache, Interface and WTF if you are resetting, so you can rename them back. Deleting WTF drops every saved setting.

Sources

Wowhead: Turtle WoW shutdown (prior research record; direct revisit restricted)
OctoWoW
Warperia
DBM backport
Grimfall