So you're still running Windows 8 (or 8.1, maybe?) and it doesn't work any more because it needs activation. That's the situation indeed.
If you can't go back to the backup you made before upgrading (maybe you didn't make one?) and system restore doesn't work either, you have three options:
1. Call the free phone number Microsoft has for activation issues: https://support.microsoft.com/en-us/kb/950929 might be helpful to find it.
2. Go back to factory conditions. How to do that exactly depends on the maker, there's no standard Windows recipe.
3. Do a free clean install of Windows 10 (only today!) using the iso or stick you make via the Media Creation Tool and your Windows 8 license key.
I am running Win8 Pro. I tried to install the free Win10 upgrade and it froze. I had to do a hard shut down to get my computer running again. Now, none of my tiles are working, it is telling me I need to activate Windows.....Error Code: 0xC004F074. What can I do to fix this?