White screen on back end (not a stackideas issue, but can anyone help?)

Hi all,
not a Stackideas issue, but it seems to be affecting the EasySocial part of the site.
This morning Joomla updated from 3.7.5 to 3.8.0. During the update an error happened.
No, I did not backup the site (I know, I know )

The site itself is working ok, apart from trying to log in (which uses Easysocial).

The back end however was giving me the header and nothing else after login. The tab on the browser had the following message (although no error was displayed on screen) - "Error: 0 Class 'Joomla\CMS\tree' not found"

My hosting provider can't help, and said they noticed I had V7 in .htcaccess which they switched to 5.6. I have no idea what that is, but now it's giving me a 500 error when I try to login instead (also on the front end)

Now I imagine something has been corrupted when the update failed.

My question is, what is the best way to repair the installation without damaging the content?

Again, I appreciate this isn't an Easysocial issue, but I'm hoping someone out there knows how to do this and can tell me how in 2 easy steps (fingers crossed) rather than me make it worse :/

Thanks for even looking, and any help is massively appreciated!