Welcome aboard, Maddy. I'm presuming you have an Intel box running Linux somewhere handy. It's much easier if you do. I'd start here as far as directions for getting the USB Console up and running:http://www.openplug.org/plugwiki/index.php/Setting_up_Serial_Console_Under_Linux
Make sure you have a solid mini-USB connection to the Plug:http://plugcomputer.org/plugforum/index.php?topic=454.0
and when you're able to reliably talk to your Console over the USB mini-plug interface, read the following carefully if you want to upgrade your Plug's OS:http://www.openplug.org/plugwiki/index.php/SheevaPlug_Installerhttp://plugcomputer.org/plugforum/index.php?topic=355
There are other ways to install a new OS, but I think Rabeeh's installer is the most comprehensive and straightforward. I'd just make sure you can access the files on the thumb drive before running the installer in earnest. Otherwise, you'll be left with an OS-less Plug until you figure out what is wrong. Read topic 355 carefully, as several of us have posted our learning curves there. And if you have questions, just ask.
Good luck, and enjoy your Plug.