Your data should all still be there, but diagnosing what the status of the OS is with the tiny snippet of information above is near on impossible.... but here goes...
I suspect you'll have lost power in the process of updating some dependent bit of dnf, hence the error above, you're statement of 'after upgrading' is mostly wrong, 'in the process of upgrading' I suspect is more accurate.
I presume the system boots to login with the above errors, is it minimal, server or something else?
First up check if rpm still works, if you get a list of rpms from a 'rpm -qa' you might be alright as you should be able to then (re)install the dnf bits and proceed to do a bunch of cleanup. Be warned, it's not for the faint of heart.
The other option would be, assuming the system has booted, to backup your data and start with a new install, with Fedora 28 beta almost done I would go with that so you have one less upgrade to do later ;-)
Reply with more answers to the above and I can likely give to you phase 2