FYI, since this was a cause of consternation for me since yesterday:

1. you are using gmail
2. you are using offline mail (via Google Gears)

then there is a bug in the launching of the new Google Buzz that causes gmail to malfunction:

The "Compose Mail," "Inbox," "Outbox," etc. under the GMail logo have disappeared. In their stead is a non-functional "More" button.
The solution:

1) Disable Offline Gmail (either at the loading screen on the bottom right, or via your Settings)
2) You'll get a spash screen introducing you to Buzz. You'll now see Buzz in your left pane.
3) Re-enable Offline via your Settings.