Have all users close QuickBooks and log out of the server. The server admin will need to follow these steps:
1. Open Task Manager on the server (or services.msc)
2. Go to the Services tab (you may need to choose "More Details")
3. Click the Name column to sort the services by name
4. Scroll down to the services starting with the letter Q
5. Right-click and restart the following services
QuickBooksDB## (the ## will be your QuickBooks year plus 10. For example QuickBooks 2017 is QuickBooksDB27)
You should now be able to open the company files again.