I'd be happy to help you find where your customer list or invoices went.
You may consider checking the Audit log first to see if those list has been deleted. If there's any data that shows that it's positive, you can ask one of the users who might have done this and enter it manually. If there's none, I'd suggest performing the browser troubleshooting method.
You can begin by using an incognito browser. This is done to verify if this is a data-related issue. There are times that the accumulated data in your browser becomes outdated over time and needs to be refreshed.
Here are the keyboard shortcuts to access a private browser:
Google Chrome: Ctrl + Shift + N
Mozilla Firefox and Microsoft Edge: Ctrl + Shift + P
Safari: Command + Shift + N
Now if you're able to see your customer list and invoices when using an incognito browser, it's confirmed that this is a data issue. I'd suggest going back to the regular browser and clear the cache. An alternative way is to use another supported browser.