I felt like this when my next door neighbour left after 5 years of living next to me and DH - and our DC. My DC were several years older than her son, and they were like a little trio of comrades for 5 years.
Had joint birthday parties (as all 3 birthdays were between 5th and 11th of March,) went to Halloween parties together, played together on the neighbour's trampoline and in our swimming pool, had BBQs (with the 3 kids, and me and DH, and my neighbour and her DH,) and went to playbarns and parks with her and the kids.
We had 7 or 8 beach days together, and had Christmas day together twice too, as her DH's family was 200 miles away, and they only went every other year (her parents passed away several years before her son was born.) She was like a sister to me. Whilst our kids played, we talked for hours. We had lunch together too, and went for a night out at the pub (or for a meal,) 2 or 3 times a month. Sometimes just us two, sometimes us two with our DHs. (Occasionally with the kids too!)
Then she announced one day - 5 years after moving in - (when my DC were 9-10, and her son was 8,) that they were moving. I was devastated. It was such a wrench, and I felt bereft. She and her DH and son were moving to Southern Spain.
Needless to say, I never saw her again. She wrote 3 or 4 times, and sent pics of her home there, and kept in touch for a year or so, and then just stopped writing. Never heard from her since. I was so sad. 😢I wrote around 10 times over the following year and never got a response. And I never knew why. I guess she made new friends and forged a new life and I don't hold it against her, but it hurt me that she stopped the contact.
It's been 20 years since I last saw her, and I have looked for her on Facebook many times, and Linkedin, and Instagram, and Twitter, all over the place. Married name and maiden name. And her son's name too. (And her husband's.) Nothing. It was an amazing 5 years with her - but I was so sad to lose her. 
(Haven't looked for her for about 8-10 years, but this thread has just reminded me of her.)