If you're no longer on someone's "added" list, and that person has disappeared from your own contact list as well and you have not deleted him yourself, what has happened? Have you been deleted or blocked or...?

Thanks!

Contacts shouldn't just disappear, probably something went wrong while loading you contacts list or you accidentally deleted  that contact.

i'm sure i didn't delete him myself, i would never do that but i believe he had reasons to want to delete or block me...
i'm now back on his list actually, and he's on mine again. i got back on it after we had exchanged some e-mails. but i didn't get a "permission to add" window, is that normal?

by the way, another contact has also disappeared from my list, and I from hers. she also had reason to delete me (she's the sister of one of my favourite movie stars, i don't even know her, it was not nice of me to bother her in that way). she's no longer on my list either, and i haven't deleted her!

Go to tool > options > privacy.
add those contacts back by right-clicking on the 'disappeared' contacts and choose add to contacts. The option add to contacts will appear if the contact is really disappeared.
There're some possible reasons behind the disappearance of your contacts on your contact list and your friends' contact list. And one of which I believe would be that, what Ezra said, your contact list didn't load properly.