Let's perform some browser troubleshooting steps and get your search function working fine again.
You can try to access your QuickBooks Online account through an incognito browser. Feel free to check below for the keyboard shortcuts based on the browser you're using.
Google Chrome: Ctrl + Shift + N.
Mozilla Firefox: Ctrl + Shift + P.
Internet Explorer: Ctrl + Shift + P.
Safari: Command + Shift + N.
If this works, it means that you need to clear your browser's cache to offload some data from your browser. This way, your browser would get optimized and we'll be able to permanently fix the issue.
If you get the same result, let's try using a different browser, then try searching for a customer again. To add, all inactive customers will not show up on the search page.
If the problem persists, I'd suggest contacting our QuickBooks Customer Care. This way, they can investigate further why you can't search your customers and help you from there. To reach us, please follow these steps: