I went to look back at a customers details today and ALL of my stored customers have vanished yet invoices and quotes seem unaffected! What's happening there?

Hi sales272, thanks for posting on the Community.

We don't currently have any reported issues with viewing customers in QBO, and I'm unable to replicate this on my system. 

To check for any browser issue that may be affecting the display, please try opening QuickBooks in a different web browser or incognito window. If you're able to view the customer list with either of these options, consider clearing the cache on your regular web browser to fix.