If someone I love hasn't written to me for a long time and suddenly says hello, and I want to express my reaction in a reproachful way, I might say something like:
"Oh, hello there. It's been quite some time since I heard from you. I must admit, I was starting to wonder if you had forgotten about me completely."
The tone in this response subtly communicates disappointment and a hint of reproach, expressing that their lack of communication was noticed and affected the relationship. It leaves room for the other person to explain or apologize if they have a valid reason for the silence.