Premiere remembers the last worksapce used. I made my the main Premiere Panel on the left monitor (the one with the main taskbar, so I couldn't control anything in Premiere) totally disappear the other day. Try adjusting your monitor resolutions from desktop... I went down to 900x600 resolution, then opened up premiere and moved other panels around to reveal the missing one. I dragged that back over to the left monitor and maximized it, then quit, and changed resolutions back to normal. Worked. I use a PC though, so not familiar with Mac.
You might also look up what keyboard shortcuts affect workspaces and panels...
Good luck