Hello, @Kirsten26.

It's great having you here in the Community. I hope you're enjoying the day so far. 

I wanted to clarify, is the customer in question supposed to have an open balance and you're not finding the invoice, or is the open balance a mistake? 

While I await your response, let's try some basic troubleshooting to the company file. It sounds like you may have data integrity issues causing the open balance to appear. Also, this can cause a missing invoice. I've included some troubleshooting steps below. 

Let's go ahead and resort to your customer balance. This helps you fix the incorrect balance showing on the lists. Here's how:
 

1. Go to the Edit menu and select Find.
2. Click on the Advanced tab.
3. From the Filter list, tap on Name.
4. Click the Name drop-down arrow, then press Ctrl+L on your keyboard.
5. Select the Include Inactive checkbox. 
6. Hit the Name drop-down arrow, then tap Re-sort List.
7. Select OK.
8. Close and reopen your company file. This ensures to reflect the changes after resorting to your company file.

If you're continuing to have issues, let's run the verify rebuild. This can help fix data-related issues on a company file. Here's how:

1. Go to the File menu.
2. Select Utilities.
3. Click Verify Data then select OK to close all windows.
4. Then, go back to the File menu.
5. Select Rebuild Data under Utilities.
6. QuickBooks will ask to make a backup before rebuilding your company file, select OK.
7. Once the rebuild is done, click OK.

I'll be sure to keep an eye out for your reply and look forward to chatting with you soon. Take care!