What is this? latex error iso-8859-7.def

[Declan O'Byrne]

I have this error, which prevents my document from doing anything. The
same document worked fine with lyx 1.5.1 on Ubuntu 7.04. Now I'm
running Ubuntu 7.10. I wonder if the problem is related to texlive,
and whether there is some package I should add?

Thanks for any pointers,

Declan