When a relationship goes cold, most of us search for the moment it broke: the argument, the cruel word. Often there isn't one. Nothing bad was done. Something good just stopped being done.
Causes and effects don't always sit close together. The cause here is quiet: the call not made, the thanks not said, the attention that went to everything else first. Months later the effect arrives as distance, and because it's so far from its cause, it feels like bad luck, or their doing. They may well have their share. But a relationship runs on what each person puts in: time, attention, regard. Stop putting it in, and it stops giving back.
That's uncomfortable to admit, because there's no villain, and you're part of the story. It's also the hopeful part. If your quiet absence helped cause this, your quiet presence can help undo it: going back with some humility, saying plainly what you missed, and starting to invest again.