Why did my upgrade to Win10 fail? I.E. where are the error messages?

I tried the Win10 free update (for users of accessibility features) today on my old Win7 desktop system. I walked away while it was doing its thing because I knew it would take awhile. I checked it from time to time and it seemed to be moving along. But when I returned to it a few hours after it started I was looking at a Win7 login screen. So I thought after I logged on there would be a popup or message of some sort telling me what had gone wrong. Or at least tell me where to find a file of error messages. Nothing. Nada.

Is there a file somewhere which will contain error messages telling me what went wrong?

Thanks, Bob

Sometimes when you open windows updates and attempt another upgrade there may be information about the failure with an error code.
If there is no information displayed then you may have to look at the Panther logs.
The Panther logs by default may be hidden.
They are full of codes that need to be deciphered or decoded.

You can use file explorer > click view > check hidden items.
Then enter each of the search files and post the results into the thread using one drive or drop box share links.

Alternatively you can use software that will create an index of all files on the computer and attached drives.
Instead of waiting for search results they will appear immediately with this software.
The more characters that you enter the narrower the search results.
Everything or everything search: voidtools

These are some of the files that you can search for and if found post them into the thread with the one drive or drop box share links. It may take a lot of time to decode the logs and sometimes it may not be possible to find the cause.

C:\$WINDOWS.~BT\Sources\Panther
C:\Windows\Panther
setuperr.log
setupact.log
Setupmem.dmp ..........[$Windows.~bt\Sources\Rollback\setupmem.dmp]
setupapi.dev.log ..........[$Windows.~bt\Sources\Rollback\setupapi\setupapi.dev.log]
BlueBox.log
$Windows.~bt\Sources\Rollback*.evtx

C:\$Windows.~BT\Sources\Rollback

BlueBox.log
$Windows.~bt\Sources\Rollback\setupmem.dmp
$Windows.~bt\Sources\Rollback*.evtx
$Windows.~bt\Sources\Rollback\setupapi\setupapi.dev.log

Setupmem.dmp
setupapi.dev.log
Event logs (*.evtx)