QB Issue Resolution:

It is likely in the help.  The answer is to choose the Customers->Customer Center menu.  At the bottom right next to the word “View” is a menu. Change that to say ALL customers.  Now the inactive ones will show up in the list.  Click on an inactive customer and choose Edit->Make Customer Active.

Resolution for Issue 'How do I make an inactive customer ACTIVE again? There is NO answer or option for this on the site!!' available: Yes (Solved).
Source: Intuit Community forum.

How do I make an inactive customer ACTIVE again? There is NO answer or option for this on the site!!: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for Center menu
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Error Recovery for Center menu [Canada]
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