Ahh, you've met (another) bug of ICQ...
Hard to say... I don't use ICQ for couple of years (with a small break), but considering people's comments here and there the program is so buggy, that is believed to be able to infest your house with roaches...
The contacts list is kept on the server server. If it disappeared because of server bug, it's likely they will use backup and your contact will return to their place.
On the other hand, if somehow your ICQ "told" the server to flush the whole list, you should better start adding them once again