If a customer is made inactive and his account is created as supplier which should have been, will the data already entered will be lost and re-entered in Quickbooks?

Hi there, @faiz-faizco-com.

Thank you for posting here in the Community. I'm here to share some information about making the customer inactive in QuickBooks Online (QBO).

Making the customer inactive is also deleting the customers under the Customer list. If you make it inactive the name won't appear on the list but the transaction will remain on the report.

However,  you can still recover the data by making the customer active.

Here's how:

1. On the left navigation bar, click Sales.
2. Go to the Customers tab.
3. Find the customer.
4. Press the name to open the profile.
5. Choose Make Active in the upper-right corner. 
   
   

I've also attached an article you can use to learn more about handling customers, like how to add a sub-customer, as well as some steps to merge them in QuickBooks: Add and manage customers in QuickBooks Online.
 

Additionally, you can visit the following write-up: Sales and customers. This will provide you with links on how to manage your company income as well as resources about other customer-related concerns.
 

Fill me in if you have additional questions about recovering sales transactions in QBO. I'm always here to help. Take care always.