Is there a report to get the total amount of money collected over a selected time period?

Yes, there is @UCBilling!

In QuickBooks Online, you can pull up and Customize one of your sales reports to identify the total amount of money you've collected. I'll be here happy to help you accomplish this.

1. Go to Reports.
2. Scroll down the page and look for the Sales and Customers category of your reports.
3. Select Transaction List by Customer.
4. Click Customize.
5. Under Report Period, select the date base on the transactions you wanted to review.
6. Below the Filter menu, choose Transaction Type.
7. On the Transaction Type selection field, select Payment.
8. Click Run Report.

In addition, here's an article you read to learn more about pulling up and customizing any of your reports: How to Get the Most out of your Financial Reports?

On top of that, I've also included this reference helpful with the steps needed in understanding how stable is your business's financial status: Setting up Reports to Customize, Run, and Share in QuickBooks Online.

Don't hesitate to post again here if you have other questions or concerns with QuickBooks tasks and navigations. I'm always around happy to help. Take care and stay safe!