Why do my sequences keep going missing?

I am working on a music video for a client and the video is being edited into 2 parts, therefore, I am using 2 different sequences. I'll be working on one sequence and once I try to switch over to the other one, it will be gone from the timeline and in my project panel. This has never happened before and it is extremely annoying because every time it happens I need to create a new sequence and re render everything (which takes almost 2 hours because of all the effects I'm using and not having the best amount of RAM space available) before I can get back to work. Why does this keep happening and how can I stop it?   In case it matters I am working with 4K footage and I am using the latest version of premiere pro on a MacBook pro