Blank page after upgrading to 2.3

I upgraded from OC 2.1 to 2.3. I didn't overwrite the config files. When I go to (my store).com/install I just get a blank page.
I have now restored my old files to 2.1 as I could t have my site down.
Does anyone know why I am getting the blank page?nothing loads at all when I go to install page to continue the upgrade.
I do currently have a third party template on my website. Any help would be really great.

Thank you.

You should be getting some error(s), if you can enable error display and tell us what comes up, that helps.

Before upgrading disable the theme (go back to default), disable third party ocmods and vqmods. then carefully follow the instructions and read forum for any known issues and their resolutions. Especially important for the 2.3 upgrade was to make sure you save and check all the settings in Admin I believe (check the readme).
Then you can turn back on the theme and mods one and a time and check they work.

@simonpieman:
probably too late for good advises, if he already screwed up, as it looks !
Ernie