Probably being a noob idiot but...

[Colonial]

my left menu (the list of commanders) just disappeared.

is there a shortcut or something to bring it back?

[ExHeretic]

There is key to hide/show the commanders menu thingie. Pushing ? in game will show list of keyshortcuts... (? means the ? key from ur keyboard so its usually + and shift). Many nice keys there :)

[sansanjuan]

my left menu (the list of commanders) just disappeared.

is there a shortcut or something to bring it back?

Never heard that one.. unless you have inadvertantly clicked on a prov with no commanders. You might try f1 and then click on a commander there to see if the menu returns.
-ssj

[Colonial]

got frustrated and deleated the savegame. It was in the first year anyway. If it happens again I will try the F1 trick...

[Calahan]

my left menu (the list of commanders) just disappeared.

is there a shortcut or something to bring it back?

Did all of the User Interface disappear, or just the commander menu?

[Colonial]

just the commander menu

[Redeyes]

Did you attempt to exit the game and starting it again?

I had this problem when playing at certain resolutions. Playing it in windowed-mode mostly solved it for me. The commanders tended to disappear for me after having been in the army setup menu.

There's an unrelated problem where the whole UI disappears, but it's related to the copy protection.

[Colonial]

It is true that when I closed and re-opened the game the problem was gone. but as I've said I was playing a different save game by that point...

[Radio_Star]

The commanders tended to disappear for me after having been in the army setup menu.

This happens to me pretty often while playing the first turn after loading a saved game. Closing dom3 and restarting fixes the problem.

[Colonial]

brillient.

If it ever happens again I will try that.