Hello Martin,
Welcome to Microsoft Community.
I am sorry to hear that you seem to have lost all Outlook 2013 information after upgrading to Windows 10.
In order to resolve this issue we need some information. Let us know the following:
Outlook information is stored in a PST file format if you are using a POP3 (POP/SMTP) email account and in case you are using an IMAP, MAPI or Microsoft Exchange account the data
is stored in an OST file which can easily be accessed again – by reconfiguring/setting up the email account again in Outlook 2013.
Check what was the type of email account you had setup in Outlook 2013 prior to upgrading to Windows 10?
In case you had setup a POP account then you may check for a folder called
Window.old on the C: drive (or your default drive on the computer prior to upgrade). On the Window.old folder check navigate to the following location and check if you can find your old PST file:
C:\Users\[USER_NAME]\AppData\Local\Microsoft\Outlook
Check the above and let us know the result.
Thank you.