Hi OP.
I am in a similar situation... in reverse.
I was friends with (we'll call her Sue) for 10 years. Almost 4 years ago, Sue was diagnosed with cancer. I was an amazing friend to her.. as well as holding down a very demanding job with anti-social hours, I attended appts with her (changed my shifts at work to facilitate), house-sat when she was admitted to hospital, done grocery shopping for her, spend a considerable amount of my free time with her, at her place either after work or on my days off. When I wasn't physically with her, we would text/speak on the phone. Looking back, I was burning the candles at both ends a bit and putting her before myself, but I wanted to be there for my friend. If I'm honest, the level of emotional support she needed from me was quite exhausting at times, but she is a lovely person and I know she would do the same for me.
However, my circumstances changed.. I had a car accident which made me less confident about driving for a while, so I for a period, I was unable to drive over to Sue's as often as before. I then aggravated an old injury which caused me constant, daily pain. This began to really get me down. I suffer from anxiety and depression which can be quite debilitating at times. I have learnt to put on a mask. Other than going to work and barely functioning, I struggle to read/reply to texts and maintain a regular level of contact with friends. I struggle to read/reply to emails. I lose the ability to focus and concentrate.
I did actually explain this to Sue at the time and apologised for my lack of contact.
A little time passed and my injury improved, as did my mood, but I was having a bit of a mental battle with something that happened in my early 20's that is not common knowledge to my friends. I was referred by my GP for counselling, which I took up. I opened up to my friend about this matter, which was incredibly difficult for me to do. And throughout my counselling, I still saw her, spoke with her and texted her, but not at the same frequency as before. I'll admit that I found if hard to maintain contact with her at this time, but I felt guilty about being a rubbish friend, so really pushed myself.
My mood improved somewhat after counselling and I was in a better place. However, I then met someone.. (we'll call him Tony). Tony was a wonderful man who swept me off my feet... and then the emotional abuse started. I was trapped in an emotionally abusive relationship for 8 months. This is not something I ever imagined would happen to me. I was cut off from friends and family and very isolated. I lived off nervous anxiety the entire time. He destroyed all my self-esteem and self-confidence. I didn't recognise the person I had become. I barely saw/spoke to any family/friends. I finally found the strength to leave and he then tormented me (and still does!) by stalking and harassing me. It was an extremely difficult time in my life. When I was able to, I started getting back in contact with friends and family. Some I was able to confide in about the abusive relationship.
By this point, my friendship with Sue was already strained. But, I still made an effort to pick things back up and made arrangements to meet, and checked in on her via text.. but I felt her response to my efforts were ..a bit cold. I did tell her all about Tony, so she is aware of that horrendous time in my life.
I met someone new (we'll call him John). He genuinely is an amazing guy. The love of my life! My mood greatly improved, I was in a much better place. I continued with contact with Sue, albeit a shadow of the contact we'd had many years ago, but we had obviously not been in as regular contact as we once had, so it was difficult to jump straight back into what we had before. Anyway, later that year, my role at work changed and I was thrust into the deep end with no support from my line manager. I struggled.. badly. All my anxiety returned and again, I was very barely functioning and wasn't able to maintain friendships. I was diagnosed with work related stress and depression and was signed off sick. When I was well enough to go back to work, I did. Again, I apologised to Sue for my lack of contact and explained the circumstances.
More recently, over the past year, I have tried to get our friendship back on track. I'd suggested a few times that we meet for a socially distanced walk etc. She is well now (in remission) and is someone who completely throws herself a work and friends.. so she is busy and hard to pin down, so I gave her plenty of opportunities for when I am free to meet. She never got back to me with any dates, just replied and told me she'd check her diary.
I sent her a text in November asking how she is and that it would be lovely to see her.
In response, I got quite a shocking reply from her in which she said "you have definitely pushed me away since my cancer diagnosis and treatment". I was very upset by this comment and somewhat offended. As mentioned earlier, I was an absolute rock to her when she was diagnosed and going through treatment. I then had my own life issues to deal with which made it harder for me to maintain a level of friendship she was used to. I did my best throughout to explain all this to her and to maintain at least some level of friendship. I honestly thought she understood that, but it is now apparent that she believes I actively and intentionally pushed her away - which is far from true.
I know these circumstances aren't exactly the same as yours, but please hear your friend out. You have no idea what she has been dealing with in all that time and she is clearly trying to make an effort - which can be really hard after not being in contact for some time. She probably feels guilty and perhaps anxious about what you might have to say to her about her disappearing for so long. But, we were good friends at one point, so at least give her an opportunity to explain.