Let me show you how to restore a deleted customer, Philippa2.
In QuickBooks Online, you can only make them inactive which means we can activate them again anytime. You can follow these steps:
- Go to the Sales menu and select Customers.
- Click the Settings, small Gear icon beside the Export button, and check the Include inactive box.
- Find the inactive customer and click the Make active link under the Action button.

You can also use the search bar if the customer doesn't appear on the list. I've added these links on how to handle customer and transactions:
Shoot me a reply if you have more questions. Stay safe!