Recently (around the July 13) AOL made some changes the software running the ICQ servers, making it impossible for users of old and alternative clients to log into (and therefore use) ICQ.
If you are running Kopete, a fix for this problem is already in SVN (where Kopete is developed). Unfortunatelly there is no new version of Kopete and Kubuntu has not (yet?) distributed updated packages either.
There is an unofficial package by djclue917 up on kde-apps.org.
To fix your setup (and update your version of Kopete as an added benefit), follow these steps:
1) Download the packages kopete_3.5.3+kopete0.12.0-0.4_i386.deb and libortp_0.7.1-0_i386.deb from kde-apps.org to your Desktop.
2) Open a Konsole window (where you can type commands). Don't panic, it's not hard!
Hit alt-f2, type "konsole" in the window that appears and hit enter.
3) In the window that appears type the following series of commands (written in bold italics):
a) cd Desktop (change to your Desktop directory, where you downloaded the package to)
b) sudo dpkg -i libortp* (tell Kubuntu to install the package Kopete needs to be installed)
c) sudo dpkg -i kopete* (tell Kubuntu to install a new version of Kopete)
4) Done! You can close Konsole now if everything went according to plan. If there was a problem, rest assured, it is very unlikely that his broke anything.
Remember: Get yourself an account with jabber.org. Jabber is a free instant messaging protocol (and much more), which guarantees that you won't have to put up with the bullshit of arrogant companies again!