This has happened to the last three chats that I created. I prompt/create using the default ChatGPT (i.e. no plugin or anything from the store), create lots of images and the next day I cannot find the chat in my history anymore. If I open my browser history I still see the title of the chat and when I open the URL I get forwarded to chat.openai.com and a banner briefly appears saying
Unable to load conversation
and then the slug/id of the chat that I’m trying to load. The chats also do not appear in the archive. All other chats that I started are just there. This issue started two or three days ago. Chats with images from last week and older are still in the history, but newer ones disappear the next day.
Two out of three conversations were quite long, 20+ images, one conversation only had one or two images.
hope anyone can help out resolve or recover my chats 
thanks!