Hi,
Make sure Windows server 2008 have installed latest updates. Then install Windows server 2012 installation media to perform a In-place Upgrade.
If the upgrade stuck in Setup is starting, this can be caused by a software and a driver compatibility issue .
you can try to delete the non-essential third-party applications and update the drivers before the upgrade.
Hope this helps and please help to accept as Answer if the response is useful.
Best Regards,
Carl