How to delete an existing SQL Server

[Theet]

It could be that you can't login any longer to an old server, because you don't have access to it or the files and user has been removed from the server.
In that case you can delete the old server and add a new one.

Delete "old" server:
1) Go in Microsoft SSMS to "View - Registered Servers"
2) Click on "Database Engine - Local Server Groups"
3) Delete the specific server

Add a new server:
1) Go in Microsoft SSMS to "View - Registered Servers"
2) Click on "Database Engine - Local Server Groups"
3) Click with the right mouse button on "Local Server Groups" and choose the option "New Server Registration..."
4) Give the new server a name and type for Authentication the default sysadmin user name (e.g. default Windows login)
After that it should be possible to work again with SSMS en the new server