Two likely cases, assuming I'm the one not saying "hi":
- I forgot your name, and given the relative familiarity we should have, it's just too awkward in context for me to approach you without revealing that I forgot… I suck that way!
- I'm looking at you, thinking about my little girl's stomach flu, or my 2.5 year unsuccessful job search, or the hole in my roof[*], and I'm just not seeing you're there. I know, our eyes are locked, but I'm not looking at you – more like through you, but so much so I don't even perceive that there's a person there. This happens quite a bit, and not just to me.
Anyway, the best way in either situation is to do an authentic "double take", then rush me like we're old friends. Just assume I've forgotten who you are, and give me your name – not in a condescending way, but with the humility of someone who really thinks they aren't memorable, but who are clearly so awesome it can't possibly be true. I, as well as any bystanders, will be truly impressed with you.
Keep in mind, I may be with someone, or preoccupied with other thoughts – but executed correctly, this type of interruption will be more "palate cleanser" than "disturbance", and a great first step towards seeding another plan together, when we both have more time.
[* No, really:]