There are times when QuickBooks online is poorly configured, Oliver. There may have been a glitch in your QBO, causing your invoices and estimates to disappear.
In the meantime, I suggest using a private window that doesn't store files. Browsers use cache and cookies to load web pages faster.
To open an incognito window, you can refer to these keyboard shortcuts:
Google Chrome: Press Crtl+Shift+P
Microsoft Edge & Mozilla Firefox: Crtl+Shift+N
Safari: Press Command+Shift+N
If you still can't find your transactions, I suggest utilizing another supported browser. It will help us identify if the issue is specific to your current browser.