Hi, @MANOOJANOO786.

When you switch to Single-User to Multi-User mode, do you see any specific error messages, such as H202 or H505? if so, you can visit this article to learn how to troubleshoot this issue: How to fix Error H202 and H505.

If you can't open Database Server Manager or see an error, we recommend you restart it on your server computer (the one that hosts your company files). To restart, follow the steps for your current setup.

If you installed QuickBooks Desktop and Database Server Manager

1. Open QuickBooks Desktop on your server computer.
2. Go to File, then select Open or Restore Company.
3. Select Open a company file. Find and open your company file. Then click Next.
4. Choose the Open file in multi-user mode checkbox, then tick Open.
5. Sign in to your company file.
6. Go to File and select Close Company/Log off. This closes your company file on the server.
7. Open the Windows Start menu.
8. Enter “Database” into the search and open QuickBooks Database Server Manager.

If everything works, you're good to go. If you still see an error message, you can proceed with the next step: If you only installed Database Server Manager.

Let me know how it goes, and feel free to return to this thread if you need further assistance.