Work on individual character's stories, or the campaign's history.
When people know that the main game isn't working, but you don't want to lose momentum, make sure you're supporting the "energy" (mental investment) that players have with the game. To this light, the remaining players have some options.
Option 1: A game of microscope
Play a game of microscope (trivially learnable in 15-20 minutes) to detail some aspect of history of the campaign world. While near-history is probably out, It's worth looking at some topic that has an immediate bearing on the plot. To this end, establish that the facts learned from microscope have been repeated in a bard's tale, and may or may not be true in the game. What's important is that the work produced is one of the accepted histories of the time, and so generally contains more fact than fiction. With this proviso, the GM can simply become another player. Worst case is that you spend an amusing game playing what-if with thoughts towards your world. You maintain some focus and have an enjoyable evening. Best case, you give the GM lots of new ideas and trivial facts to work into future encounters and have a much richer world and history to play with.
Option 2: Flashbacks for character backstory
Either using 4e or some minimal system (like minimus), play out a character's backstory. Again, using the "this is how a bard would have told it." or "this is generally what the [local area] thinks that happened." you can explore a character's past life and thereby enrich the world.
Option 3: Ensigns on the enterprise
Looking at this very recent post on the "Guest GM" experience, Gnome Stew suggests how to make a guest campaign in the guts of the main one. Roughly speaking, have a guest GM run a game based on the lives of some underlings of the primary characters. The game will be much lower level and should follow the events of the primary story, but from a different (and lower-level) perspective. Again, you maintain focus and enrich the primary world through these actions.