To reopen the company file in QuickBooks, first stop sharing it in Quickbooks Server...

Let's perform some troubleshooting steps so we can fix the error, Pasion.

Normally, when we talk about connection issues, this is caused by an incorrect, corrupted, or even irresponsive database server. That's why it's asking you to close QuickBooks. 

You can restart QuickBooks. This will clear the current issues in the background.  After that, run the Quick Fix my Program from the QuickBooks Tool Hub. To do so, follow the steps provided below:

Step 1: Download and install the QuickBooks Tools Hub.

The QuickBooks Tool Hub helps fix common errors. You'll need to close QuickBooks to use the tool hub. For the best experience, we recommend you use Tool Hub on Windows 10, 64-bit.

1. Close QuickBooks.
2. Download the most recent version (1.6.0.3) of the QuickBooks Tool Hub. Save the file somewhere you can easily find it (like your Downloads folder or your Windows desktop). Note: If you've installed Tool Hub before, you can find out which version you have. Select the Home tab. The version will be on the bottom.
3. Open the file you downloaded (QuickBooksToolHub.exe).
4. Follow the on-screen steps to install and agree to the terms and conditions.
5. When the install finishes, double-click the icon on your Windows desktop to open the tool hub.

Note: If you can't find the icon, do a search in Windows for QuickBooks Tool Hub and select the program.

Step 2: Run the Quick Fix my Program from the Tools Hub,

1. In the QuickBooks Tools Hub, select Program Problems
2. Select Quick Fix my Program.

However, if you're still experiencing the issue, proceed to Solution 3 in this article. Error Message: Connection Has Been Lost.

In addition, you may want to know more about QuickBooks in case you experience issues, I've added this resource for your reference: Get Started With QuickBooks Desktop.

Please get back to us if you need further assistance. We're always right here to help you.