Hello smounche Welcome to the Ten Forums!
I have one immediate question for you as far as do you have any other hard drives present during or after the initial upgrade to 10 by chance? The problem with the 10 installer is that it can get confused especially during the initial set up and place boot files, temp install folders in the wrong locations! If you have any other drives be sure to see those are unplugged for either a repair install or a second clean install if things come to that.
Before trying either a PC Reset or Repair install outlined in the guide for that Repair Install Windows 10 with an In-place Upgrade - Windows 10 Forums=Installation%20and%20Setup a few other things can be tried first if you are able to access the command prompt option in particular to run the Disk Check tool on the drive in case of disk errors fouling up the works. You would enter "chkdsk /r" or "chkdsk /f" at the prompt to get the Disk Check running manually there.
In the event you decide to follow through with another clean install be sure to review the guide for that option. Windows 10 - Clean Install - Windows 10 Forums=Installation%20and%20Setup
You should first however get into the Advanced options to see if one of the repair options including the Startup Repair tool can help out before taking the plunge. That's the automatic repair tool option in addition to getting the Disk Check tool a run to see what that can fix. The clean install guide also covers some things like the UEFI potential issues that can come up as well and should be reviewed to see if you did happen to miss something along the way.
Thanks for your fast reply,
i have an SSD - where the OS was first install (it was a clean install, not an upgrade)
an additional - i also a D: hard-drive - for data. (not ssd)
i will try now to unplug it - and load windows without it - and also see if i have access to command prompt.
Startup Repair tool - tried that in advance options - no success ..
That data drive is your problem then! I ran into the same thing only 10 wouldn't even upgrade at first over a cloned 7 or fresh clean install of 7 until finally taking a look at what I had missed when reviewing the clean install guide. Two 1tb storage drives were found to have the 10 boot files on one and the $Windows~BT and WS folders on the other!
With the 64bit 10 the "$Windows~WS\Sources\Windows\ sub folder is where the main "setup.exe" file is located in case you ever need to see a repair install and haven't nuked that off the drive with the DiskCleanup tool that is. But I suspect you will find some unexplained files and folders on the second drive there.
You need to redo the install with the D: drive disconnected, at least I think that is what Night Hawk is suggesting.
Here I intentionally went ahead and did just what many would do without prior warning about the 10 installer getting confused on where to put things! I suspect when you are up and running again and take a look at what is on your second drive you will find some unexpected boot files and temp folders! But that will now have to come later since there is one last chance to possibly see that first install made bootable again.
On second desktop not the first where I had left the two storage drives plugged in but on a single drive system I still didn't quite see the same routine there. Instead of immediately seeing to the upgrade over the existing 7 Pro install I split the drive up and at first saw the clean install proceed the upgrade install putting onto a second primary for the single drive case there. Within a day following the upgrade over 7 take place the previous clean install was found to have activated on it's own when I then nuked the first having the boot files and BCD store and moved the second new primary forward and expanded it to fill the drive.
The end result was the obvious of making the drive unbootable where I then tried the "bootrec /rebuild" command but forgot to first use DiskPart and select that volume to make it active. When the bootrec option failed here I simply replaced the first with the second and later restored an image I made of the 500gb drive's original 7 install to a second 1tb drive added in for the dual boot there.
Then came a laptop for more fun!
For you it may be possible to see the "bootrec /rebuild" option work as far as seeing a working BCD store created where things got scrambled up a bit having that second drive plugged in at first. You would need to boot live as you did for the run of the DiskCheck tool which showed you were looking at any volume of errors on the SSD but are seeing the same problem of scrambled boot! Boot files placed on the wrong drive most likely to be found afterwards.
With the second drive unplugged when going to run the DiskCheck tool there was so sign of any boot files and/or BCD for the Startup Repair tool to fix. And for the last chance option before seeing a second clean install take place you can give the "bootrec /rebuild" command a try. It might just work for you there at least well enough where the Reset PC option will take care of the rest. Otherwise you will likely end up with the need for the second clean install to take place now that the D drive is out of the picture.