Have you any idea for the returning error 404 from remote server?

Hi,
My Nuget server was working smoothly. Recently, I have upgraded it and received an error message.
The screenshot of the error message is as under. Have you any idea for the returning error 404 from the remote server?
It is so urgent for me to get a remedy of this issue.
Please help.
Thanks.

Installing
Operation failed.
The remote server returned an error: (404)
Not Found.
