Re: lost my Yahoo email contact list on coversion to Windows 10
The case is unclear. You Yahoo!Mail account is virtually independent of the status of your PC since your email contacts are saved in the Yahoo! servers - not your PC. Hence, your Yahoo!Mail contacts should still be intact regardless of your OS.
If you're referring to Outlook for Windows, then you should be able to bring all your contacts back by setting up your mail account again.
You may create a Windows Central account and Reply in this thread to provide additional information in case I am missing your case.