<p>CuteAccountant has a good point.
Also consider if it’s somebody you know well or not. Sometimes computers can accidently delete friends as well. If it’s a close friend, just send them a message on fb or ask them “Hey, I couldn’t see your wall/comment on your wall/comment on your picture the other day. Do you know what’s going on?” If they don’t respond, did you fight with them or something?</p>