I know you are supposed
to be able to "upgrade" from 8.04 to 10.04, but I don't know anyone who has managed this without problems (maybe they just stay silent?)
So tried installing kde-core, but it says it can't find a package by that name.
8.04 used KDE 3.x 10.04 is using KDE4.x - there's a huge difference between them.
But you can still boot, and get a login, which is good news.
I suggest you make a backup of /home
+ anything else you need (Eg /var/www
- you don't specify what sort of "server" this is) to an ext3 formatted external HDD, do a clean install of 10.04, apply the updates, then restore your data.
Off topic: You might not like KDE4 (I hate it, and have switched to gnome).
Not the best way to spend a sunny weekend, I know, but maybe someone else has a better idea.
Meanwhile, I hope you took a backup before you ran sudo do-release-upgrade --proposed
in which case I suggest you restore it while you consider your options.