This was a PC which just would not run at all after the upgrade.
I made a recovery USB stick via the advice from Microsoft , plugged this in and restarted the PC.
Note: to boot from USB you may need to change the BIOS settings. This is rather tekky and I won't go into details here. Use this search for assistance : https://www.google.co.uk/webhp?ie=UTF-8#q=bios%20change%20boot%20order or grab a passing geek.
Once booted up, I followed the Troubleshooting then Advanced Options then Start-Up Repair.
This latter claimed that it could not find anything to repair.
Feeling not too hopeful I allowed the PC to restart. To my surprise Windows 10 now started up. Evidently the process had nudged something into place.
(Obviously if a Repair solution is offered then accept it and follow instructions).
The computer was rather ponderous but I was, however, able to revert to Windows 7 thus:
Press the Start/Windows key :
then choose Settings - Update and Security - Recovery....
....and then you'll see an option to Revert to Windows 7. Click the button and follow instructions. Takes less than an hour. After that see preventing the Windows 10 upgrade to prevent a recurrence.
A laptop which would not go beyond the login stage after upgrade. This problem applied to the main user account. There was a second account which would log in OK. I tried to use the Revert to Windows 7 process above but was not offered the option because the account was not an Admin one.
To try and resolve this, I went to the Control Panel (Start-X is the quick way) and changed the user to Admin (I used the password from the main account for this).
After that I tried Settings again but the Update and Recovery option wasn't working (it just cleared the screen).
Next step was to book in Safe Mode. This is done by Start - Power then holding down the shift key when clicking Restart.
When the laptop restarted the 'Safe' menu appeared. I selected "4" (safe mode) and let the computer carry on.
After logging on this I was able to revert to 7 per the procedure above,
It's always a good idea to have more than one account on your computer anyway.
If you do find that Win10 is stuck and there is another user account then try that one as it may well work and allow you to revert.