All revenue data prior to May 2021 has disappeared from my Profit and Loss reports. Does know why this would happen and how to recover?

Let's try running the report on an incognito window,  tombirch.

There are times that an issue like this occurs when the system encounter a browser-related concerns. We can use these keyboard shortcuts to open one:

• Google Chrome: Ctrl + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Microsoft Edge: Ctrl + Shift + P
• Safari: Command + Shift + N

If it works, we can go back to a regular browser and clear its cache. We can also switch to a different supported browser like Google Chrome, Mozilla Firefox, Safari, or Microsoft Edge as alternative.

Additionally, here are some resources you can use in managing your reports in QuickBooks Online (QBO): 

• How to memorize reports in QBO
• How to customize reports in QuickBooks

Let me know how it goes adding a comment down below. I'm always here to help you. Keep safe!