I can't play Minecraft.

Whenever I join my server, I see nothing but sky. Nothing happens. Eventually I get killed by a Zombie I never saw, respawn, and there I am again staring at the sky.

When I press F3, I see a "waiting for chunk" message. Nothing happened after 5 minutes, can't bother to wait even longer. I consider the game broken beyond playability right now.

EDIT:
Nothing suspicious in either the server log or the client log.