During every training session this question comes up multiple times.  “How do I get my Project Browser (or Pr0perties, or Zoom dialog thingy) back on the screen.  If you accidentally turn off any of these they are easily brought back by going to the View tab >> Windows Panel >> User interface drop down and rechecking what you want back on.  You at anytime can bring back the Recent Files screen that is shown when you open Revit with this as well.