There are a few options to help debug the issue which can be caused by a variety of issues.
Check for Postman Updates
Restart Postman - ensure that you stop the Postman process by exiting the software. Then confirm that all Postman background processes are shutdown by checking your Task Manager. Once you confirm all Postman processes are existed, start Postman again.
Clear Postman Cache (ensure you save any modified requests/collections/environments as they will be lost) by going to Help > Clear Cache and Reload
Check for Large/Complex Scripts. If your collection contains a lot of requests, complex pre/post-request scripts it can cause the process to stall. Try a smaller collection to see if the problem persists (a collection with a single request for instance).
Run in command line (Postman CLI or Newman). Attempt to use the Postman CLI to run it in a terminal instead of the GUI. If this works, it will be likely something with the volume of scripts/requests in the collection that is causing a hiccup with the GUI environment.