Sometimes WP permalinks break silently.
✔ Fix
Go to:
Settings → Permalinks → Save (without changing)
This regenerates .htaccess with:
# BEGIN WordPress
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
# END WordPress
Instant fix.