The flip-card in my Godox 685 fell out; what to do?

If it was me, I'd just get a cheap bounce reflector that attaches to the top of the flash head with an elastic band. In fact, that's what I often use with my camera mounted speedlights or off-camera speedlights in 'run-and-gun' situations, even though the pull-out bounce cards on my flashes are all intact.

The one I use the most is the smaller of the two that came in this 2-piece kit with one small and one larger reflector. I paid about $15 for both. The larger one is a little unwieldy for on-camera use but does nicely for quick off-camera usage. I can set the flash mounted on the supplied "foot" on a table and aim it at my subject to get off axis light, shoot, and then grab the flash and move on in a matter of seconds.

If you don't want the larger one, you can find kits with two of the smaller example for about $10.

Most of the smaller ones have a white side and a silver side. The elastic can be easily flipped to either side to use whichever side you want.