upgrade to 15.10 failed - how do I go back?

Hi,
I was on ubuntu 14.xx LTS and rather rashly I decided to change my upgrade option to show all new releases and so attempted to upgrade to 15.10.
This was installing nicely, then the screen went black, there was no disk activity and it would not respond, so eventually I rebooted...
And I caot get a system back up.

If someone knows what do next with their eyes shut then that would help, as I have not much information to give.
This laptop has a cracked screen and I use a vga cable to display to a TV.
I have never seen any boot information - the TV only displays anything when the desktop is ready.
The laptop screen glows white or is off - binary!

Currently on boot, the laptop screen glows for a minute then goes off - the caps lights are responsive on the keyboard, but then there is no disk activity.
I have tried newline.
I have tried down arrow, newline, down arrow down arrow in case I am on the grub menu.
But there is no change or response.

I did originally install ubuntu blindly from a usb stick, and would do this again as there is no vital data on this laptop, but the mrs has been tidying and could have put it anywhere...

It could be the laptop has a hardware problem but that would be quite a coincidence, just after a failed upgrade.
So here I am. I just want to retu to version 14.

Any ideas - I can try any ideas later this evening.