This is a very popular request for customers, who want to know which accounts, leads, opportunities are still owned by users who are inactive.
That way they can reassign them to active sales reps.
You will need to create a new custom report type. For further information, please review the article Set Up a Custom Report Type
The following is an example on how to build a report for leads owned by inactive users:
1) Go to Setup > Create > Report Types
2) Click New Custom Report Type
3) Primary Object is Leads
4) Add in a name and description (as a best practice we suggest to use CRT in the name, example "leads CRT")
5) Store in Category Leads
6) Check Deployed if you want your users to see the custom report type
7) Click Next and then Save
8) click on the "Edit Layout" button
9) Click on "Add fields related via lookup", located on the right under the View box
10) Click on Lead owner and select the Active field (we are including the field to the report type available information,
you will be able to filter by "lead owner: active equals false")
11) Click OK and Save
12) Go to Reports, create a new report, referencing this new report type (search the "leads CRT" or the name you put as label)
Create the report with the criteria you want and remember to add the filter "lead owner: active equals false" on your report filters.
The report will look similar to: