Its like reinstalling Windows on a PC, just in case you just have a software problem and the hardware is fine. Are you still running OS.2007? Its worth installing 2008 anyway, I think.
You should backup your data to memory card first - see backup program under 'settings'. Ahh .. d'oh! you can't - sorry.
And you would need to reinstall your applications anyway. Its all a LOT easier than on Windows though.
Of course, if your hardware is bad, it might not even be possible to do this. See this forum for OS.2008 upgrade info.