This is a common problem often happens when you use an additional external monitor (An Apple Thunderbolt Monitor) as the Apple Mac remembers where you previously had the application open, and that was on the external monitor.
The problem occurs when your Apple Mac continues to remember that your application was open on the external monitor even you are not using an external monitor anymore. So the application is open but is stuck, lost, and hidden somewhere where you cannot see it.
To resolve it, you can try follow the following steps.
Go to your application and find the menu. In my example, I am trying to find my Microsoft PowerPoint screen which is hidden away - as I last opened a PowerPoint on my external monitor.
Your application should now be on your screen now.
I just did this now, which is why I am writing this guide as I wanted to share this your finding with you ;)