The lefthand side where it used to just be a list of all the folders in my lightroom catalog is now linked to my desktop where I have those folders/photos. Did I do something to change this? I'd like to go back if anyone knows how! Thanks!
It's now clear from your post whether it's the entire set of panels on left side that are missing or just the folder panel.
If entire left side then click on the little triangle I've show with red boundary in below screen shot. This will expose the left side panels.
If it's the folder panel that's missing then simply right mouse click on the header bar any of the other panels (e.g. Collections or Catalog) to open a context menu. Now click on the Folder item in the list.
Show us a screen-clip -specifically all of the open folder panel.
It is possible to "Show" or "Hide" Parent folders with mouse right-click options.
This may be part of an answer for you.