Thanks for posting here, @kim-ashleymartin,

I can share the steps to help you rectify this missing customer list issue in QBO. For now, let's see if this is a browser related problem by opening your QuickBooks in a private window.

Unexpected behaviors in the program, are sometimes affected by issues in the web browser. This is because the cache will overwrite itself and will not delete any data over time. 

Private browsers prevents cookies from being stored on your computer, making it a great place to identify browsing issues. Use these keyboard shortcuts to open a private window:

Google Chrome: press Ctrl + Shift + N  
Mozilla Firefox: press Ctrl + Shift + P
Safari: press Command + Shift + N

If it rectifies the problem, clear the browser's cache. Clearing the history or log of sites resolves most issues with the web pages.

Also, make sure you're logged on to the correct company file, in case you have multiple companies.

Keep me updated with the result, as I want to make sure this is taken care of. I'm also here if you have any questions or need help with other QuickBooks processes. Have a productive week!