Hi {boss's name},
I don't want to start the new year in a bad way, but I am offended that I said hello to you earlier in the office and you didn't say hello back. I don't know if you meant it, but it came across as rude and for me that is very demotivating.
Best,
{my name}
Context- my boss has been away for 6 weeks and today i saw her for the first time, she came into the office and said “hello {my colleague's name}” and has a nice chat with my colleague but completely ignored me when i said hello to her. This is a pattern of rudeness rather than a one off. I'm finding it hard to work at all because I'm pissed off.