You can reactivate your customer by going through the customer page, Franz4.
 
Inactive customers are hidden from the customer list in QuickBooks Online (QBO), although their transactions still appear in reports. Enable the Include inactive option to view and reactivate them.

Here's how to do it:

1. Log into your QBO account.
2. At the left panel, choose Customers and Leads.
3. Go to the Customers tab.
4. Click the Gear icon above the Action column header.
5. Check the box next to Include inactive, then search for the name of your inactive customer.
   
   
    
6. Once found, select the Make Active link from the Action column.

For a complete customer handling guide, you can visit this article: Delete customers in QuickBooks Online.
 
Additionally, you can generate and send customer statements in QBO to notify them about upcoming balances or invoices, offering options like Balance Forward, Open Item, or Transaction Statements.

Do you have any other inquiries about managing customers in QBO? You can leave them in the comments below. I'll get back to you as soon as I can.