Well, I don’t know about the specifics of the SheevaPlug (looks sweet though! :))
First, check out firefly’s log file and the syslog for obvious messages.
Don’t panic – linux lets you custimize lots of stuff, so theres no use in doing upgrades etc if something doesn’t work.
Usually that just makes matters worse. Instead take a deep breath and dive into the bowels of your SheevaPlug! 😉