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I got your back on this query, and I'll share some steps you can take to investigate this issue.
I suggest you check your Audit log page. It lists all changes made on your QuickBooks Online (QBO) company, so you can get to the bottom of it.
If your customers were deleted accidentally, you'll have to re-enter them. Otherwise, you can continue with the troubleshooting steps below.
You can also open your browser's private window. This isolates the cache, which is the common culprit for unusual behaviors in browsers.
If that works, go back to your regular browser and clear its cache. Otherwise, open an alternative browser that's available for you.
In case you encounter any other browser-related issues, you can refer to this article: Troubleshooting browser problems. It has a specific set of steps for each browser that's compatible with QBO.
Feel free to tag me in the comments below if you have other questions. I'll get back to you as soon as I can.