There are a few different things you can try to fix the issue. Feel free to go through the suggestions and see which one works best for you.

Verify the Connection Manager version

Check that the version of the Connection Manager on the server is the same as the workstation.

1. The connection manager needs to be on the same version on the server (main computer) and the workstation.
2. Type Sage 50 Connection Manager in Windows search on your desktop.
3. Click the Connection Manager app to open it.
4. Check the version on the About section.
5. Compare the two versions (server/main computer and workstation).
6. If connection manager versions aren’t matching, install the latest Connection Manager on the server.

Start the Sage 50 Connection Manager

1. Click the Windows Start button and type Sage 50 Connection Manager.
2. Double-click the Sage 50 Connection Manager app. If you have your data on a server, follow the same steps on your server.
   
   • If the status shows "Running: No", select Start Service
   • If the status shows "Running: Yes", select Stop Service, then Start Service
3. If the button for starting or stopping the service is missing, repair the installation or reinstall Sage 50.

Start the Connection Manager Service in Windows Services

1. Click the Windows Start button and type Services.
2. Double-click the Services app.
3. Scroll down to the Services list and click Sage 50 Database Connection Manager.
4. Check the Startup Type column status. 

If the status is Disabled

1. Right-click on Sage 50 Database Connection Manager and select Properties
2. In the General tab, change the Startup type to Automatic or Automatic (Delayed Start)
3. Click Apply
4. Click Start to start the service

If the status is Automatic or Automatic (Delayed Start)

1. If the Service is Starting (as opposed to Started), close Sage 50.
2. Access the Task Manager (CTRL+SHIFT+ESC) and click the Processes tab.
3. Click the Show processes from all users' button.
4. End all related Sage 50 processes, especially the Sage 50 Database Connection Manager.
    NOTE: If you're unable to end the process, try restarting the computer to clear out the cache.
5. In the Services window, right-click on the Sage 50 Database Connection Manager and select Start.
6. Ensure the status shows Started or Running.
7. Open your Sage 50 file again.
8. Uninstall The Connection Manager if it doesn't open.

Temporarily disable the antivirus and security programs

See Third-party utilities, antivirus, and firewall issues with Sage 50.

Restart the computer

1. Restart the computer or server where the data resides. 
2. Check for pending updates and install them at the same time.
3. Reopen the data file.