Your customer information may have been accidentally deleted. Let's first access your QuickBooks account using a private browser or incognito browser. By doing this, we can isolate this unexpected missing customer information for a possible browser-related.
You can use either of these keyboard shortcuts:
Google Chrome, press CTRL + Shift + N
Mozilla Firefox: press CTRL + Shift + P
Safari: press Command + Shift + N
Once logged in, let's try locating your customer information. You can start by viewing the Customer Contact List. It contains the customer's phone number, email, billing address, and other contact info for each customer. Here's how:
On the other hand, you can view the audit log to check if any changes were made to your customer's information. It allows you to keep track of your activities, where you can see who made changes to your books and what they did. This also includes the time in and time out of the user into the system. Here's how:
Go to the Gear icon.
Choose Audit Log.
Use the fields on the Filter panel to choose the appropriate User, Date, or Events filter to narrow the results.
Hit on Apply.
If your customer's information or invoices were deleted, you may need to manually re-enter it.
I'm adding these articles as an additional reference on how the Audit log feature works: