It indeed seems like you close the "Project Explorer". As Robert stated you can retrieve it by going to the "View" menu, which can be found in point 1. I founded a url for you: https://gettingstarted.mendixcloud.com/link/module/93/lecture/714 , which explains more about the "Mendix Modeler" and their names and what they do! Hope this will help you understand it more if you have some other problems in the future!
Sounds like you've closed the "Project Explorer". To get that back (or any other closed window), go to the "View" menu, and select it. If you want to reset your modeller layout to factory settings, go to "View" and select "Reset Layout..."