What the heck happened indeed. Don't panic.
First off:
CCleaner would not cause a Windows restart, either at installation or when run.
And a Windows Update is a Windows Update, it's nothing to do with CCleaner.
It's just a coincidence that you had run CCleaner shortly before Windows decided to Update itself.
So what has happened?
I suspect that what you have seen is an Automatic Windows Update to Windows 10 version 20H2, (or possibly even 20H1)
Microsoft have been 'rationing' them out over time, especially to machines that may not have been fully compatible with the Update.
There are some changes to 20H1/20H2 due next Tuesday and so more machines will now be compatible, which may explain why you got it now.
(I have a friends laptop here at the moment that says it's almost ready for 20H1/H2 but not quite yet, when it is it will automatically update itself).
Do you know what version of Windows you had before this Update?
If it was 1809 then Windows would automatically update it, even if you had automatic updates turned off.
(That's because 1809 has reached 'end of life').
So what now?
It is known sometimes for your files and desktop to go, or appear to be gone, following a Windows Update.
Again don't panic, they should still be there and you should be able to get them back.
(It's only in rare cases that they have actually gone).
The most likely explanation is that is because the Windows Update has logged you into a temporary user account rather than your usual user account.
Although as you say "logging in took some time" then I suspect that you have created, and are now using, a different user account than before. (A new one).
In that case you are now logging into and using a different user desktop and settings, different documents and photo folders, and so on.
That would look exactly as you describe. - You can't see your old documents/photos/etc. <em>because they are in a different user account.</em>
(PS. See below for what has happened to the free space on the disc).
The older account should still be there, with it's old desktop, settings, documents and photos, etc. If it is you just need to change which one you log into.
If you restart your machine do you see more than one account to log into?
If you can still see the older account then you just need to change which is the default user account.
If you can't see the old user account then there are various other things you can do; such as look for a backup of that account in 'windows.old'.
There are online articles that can help with that, but if you are not techie minded they can often be a bit much to follow.
If you are not techie minded (or don't have a techie friend to help) probably the simplest way is to just 'roll back' the Windows Update to the previous build. If you are still in time to do that.
(You may not be able to rollback if this <em>was</em> an update from 1809).
That does take an hour or two to complete.
The fact that you say a lot of previously free space has been taken up gives hope that you should still be able to roll back.
That extra used space will be full of a 'hidden' backup of the old Windows installation and files.
See this for how to roll back a Windows Feature (Major Build) update.
https://pureinfotech.com/uninstall-windows-10-2004-2020-update/
If you do that then once you have rolled back Windows make a current backup of your files, photos etc. - and let Windows try the update again to see if it can get it right this time.