I used the Media Creation Tool to upgrade mine and all seemed good.
While using it the Start menu stopped responding, I was downloading an updated Nvidia driver at the time in Firefox which subsequently failed.
I rebooted the system and received the critical error that Cortana and Start weren't working and got prompted to sign out and back in.
Did this a couple of times with the same issue, even completely powered down the system to clear RAM but no joy.
Managed to get it into mscondig and selected Diagnostic Startup but nothing seemed to happen, managed to get Windows to restart but because of the change it disabled networking, Volume Shadow Copy (so no system restore) and a number of other things.
I wondered if it was because I'd left Avast running during the upgrade, anyway I got it to roll back to 7 and will wait the prompt to upgrade.