Smart phone mistake block? Easily happens, go to view and hit block by mistake. Doesn't normally let you re add though, request has to be sent, unless its been altered
We've had that happen a few times before. I think if you accept a friend invite, then they 'unfriend' you, they can add you again automatically by sending you an invite. If you 'unfriend' them, it doesn't work. Not sure it happens all the time - maybe just an occasional glitch.