The Design Navigator (i.e layer browser including bulbs) is missing from the screen. And now I'm not sure how to enable it again.

If you wish to hide the "Design Navigator"

1. Go to menu:  Setup / Customize...

2. Select the tab "Toolbars".

3. Un-check "Design Navigator", and press OK.

Note:  If you wish to show the Design Navigator then check "Design Navigator".

If for any reason it still doesn't appear, make sure you haven't checked "Full Screen mode" (menu: View / Full Screen).  It's also possible hide the Design Navigator by simply dragging it to a zero width.  In this case move the mouse over the side where the design navigator existed and drag it back.

If you're still not sure than, go to menu:  Setup / Options..., and press Reset.