suddenly a running balance isn't appearing in my chart of accounts?

Hello, Kacey. Are you referring to the QuickBooks balance or the Bank balance? If so, you'll want to ensure that one of those options is selected in your Chart of accounts Settings to restore that column. Typically, the running balance disappears because the column has been accidentally hidden from the main view.

You can check the applied filters and select either of these two options on your Chart of Accounts page, as shown in the screenshot below:

However, if you've confirmed that it was being selected yet this column still isn't appearing, let's perform some basic troubleshooting to rule out a browser cache issues by signing into your QBO account using an incognito or any supported browser to refresh the page performance.

I'll also share this article to help you get started with your reconciliation: Reconcile an account in QBO. 

Feel free to reply to this post if you need further assistance.