Hi @Kate70,
Sorry to hear that your loved one has 'gone off the radar'. It is certainly a worrying time and your right boundaries are not necessarily always the answer. @KittyK raises a great point that there may be some comfort in the fact that the disappearances are cyclical and contact does return. It is important that during these times you find some ways to look after yourself, even though this may be the last thing on your mind you are important too.
It sounds as though your loved one disappears after a misinterpretation in a conversation which they may have taken the wrong way due to their mental health. It may be possible to patch it up when they 'reappear', however its difficult to say without understanding the context of the mental illness. You can always discuss with your loved one the conversation that took place and try to get an understanding of what they took from the conversation and if there was something that upset them. Have you tried this in the past when they have disappeared? What tends to happen when they do 'reappear'?
Look forward to hear from you 
Rockpool