Yesterday, I discovered that I had accidentally muted an important conversation. I found no obvious way to undo it. This lifehacker blog post points to the solution:
To un-mute a conversation, select the conversation and click Move to inbox. Doing so will move the entire conversation to your inbox and will remove the mute action, so that future messages are also delivered to your inbox.
Muting conversations is very handy, by the way. From the Gmail help pages:
If you're subscribed to a mailing list, you've no doubt been subjected to the 'thread that just won't die!' If you're part of a long message conversation that isn't relevant, you can 'mute' the conversation to keep all future additions out of your inbox. By using the 'm' shortcut key, new messages added to the conversation bypass your inbox so that the conversation stays archived. If your address appears in the to or cc field, though, the conversation will pop back into your inbox ready for your attention. Muted messages are not marked as read, are still searchable, and can specifically be found by searching for: