Happened to me twice, the first time it was a family we were really close to, used to go on holiday with, exchanged letters and phone calls weekly. She suffered the death of a baby at a few weeks old and I did everything I could think of to be supportive, and she seemed to appreciate it. But one day a letter was ignored, phone messages left and never returned. I was really worried. Eventually my husband managed to catch her husband on the phone, and he said she had taken against a sentence I had written in my last letter. I at once replied deeply apologising, but no response, they just shut us out of their lives.
The second time I was having regularly lunches with an ex work colleague, we had both been bullied and work, and were glad to leave, but continued to see each other. I arranged on lunch date but completely forgot, and had to rush over when she rang to ask if I was coming. A couple of months later she also forgot, it was a charity fund raiser, and she was too late to join in, so I considered we were equal on the amnesia front. I then went down with a ghastly embarrassing illness that meant I found it hard to think of anything else but how uncomfortable it was, and I left work and drove home on automatic pilot when I should have driven to the pub to meet her. She rang and I apologised profusely and said I was on my way, but she just said 'it's too late, don't bother'. I tried to rearrange but she just ignored all phone calls and emails, so I have sadly had to accept I have lost her.
Others can shrug these things off, I am too sensitive, and still would welcome further contact if the chance ever arose, it does hurt.