In the library module, left-hand panel, there are different sections or tabs. Collections is one of those tabs near the bottom out that panel. If you scroll up in that panel you should see other sections, one of which is "Folders". If that isn't showing, then right-click on the Collections section heading and make sure there are checkmarks in the sections that you want to have displayed. To return to the folder you need to go to the folders section and find the folder in your list.
Another way to return to a folder is to click in the area in this illustration:
Doing so will present a list of folders that you have recently been inside of. You can click on one of those folders and return directly to it.