I hope you are having a great week so far! I have something I would like to share that might help you bring back your customers list.
You will want to follow these steps:
1. Select Sales Tab from the left hand side menu
2. Select Customers Tab
3. Select the blue "Clear Filter/View All Hyper Link
I have uploaded an image to assist you with what we are looking for, I hope it helps!
It could also be a possible browser issue stopping your customers list from populating on your drop-down when inside an invoice. To troubleshoot possible browser issue disrupting your QuickBooks Online, we can follow the link below:
Clear cache and cookies to fix issues when using QuickBooks Online