OK. Let’s check first to make sure you have all of the required modules when running 8.1:
Download and run that. If anything is in red, you know what you need to fix.
Assuming it comes back clean, you can attempt to run the updater again. Just visit domain.com/admin/upgrade
If it says there is nothing to update, the files never extracted properly. You can download them from your client area on this site, upload them, then run the updater that I noted above again.
Let’s see if that gets you rolling in the right direction. If it does not work, we can use recovery mode to disable all your 3rd party resources as one of those may be interfering with things.