Computers default to single user mode on Mondays. Last year I was able to fix by checking a small box I had missed. Where is this box? I do not remember.

Hello there, @Kimworking.

Good to see you here in QuickBooks Community. I'm here to help and guide you on how to change the default user mode in QuickBooks Desktop. 

Are you trying to default your QuickBooks to Multi-user mode? If so, let me walk you through the steps: 

1. On the No Company Open Window, select Open or restore an existing company. 
2. Choose Open a company file option. 
3. Click Next. 
4. In the Open a Company pop-up window, put a check mark on the box of Open file in multi-user mode.
5. Click Open. 

If you need further assistance with the steps, I encourage you to contact our QuickBooks Desktop Technical Support Team. They have the tools to pull up your account and do a screen share. 

Here's how to contact our phone support:

1. Click this link: https://help.quickbooks.intuit.com/en_US/contact. 
2. Select the version of QuickBooks, click Contact Us.
3. Type-in Technical Support on the search bar. 
4. And click Get the Phone number below. 

For future reference, you can check this article: Set up multi-user or network for QuickBooks Desktop.

This will get you on the right track. Don't hesitate to leave a comment if you have other concerns about Multi-user mode in QuickBooks. Have a good one.