See who did what

• Settings → Activity Logs — a record of changes in your admin: who changed which product, order, setting or page, and when.
• Order Timeline on each order — every status change and who made it.
• Change History on each product — every field change with the old and new value.
• Settings → Email Logs — every email the store sent, and whether it was delivered.

If something changed and nobody knows why, start with Activity Logs and filter by the date.