Without logon it will be difficult to do anything with this one. If it were me, I'd switch it off, do metadata cleanup.
then confirm health is 100% and stand up a new one for replacement. I'd use dcdiag / repadmin tools to verify health
correcting all errors found before starting
any operations. Then stand up the new 2012, patch it fully, license it, join existing domain, add active directory domain services, promote it also making it a GC (recommended), transfer FSMO roles over (optional), transfer pdc emulator role (optional), use dcdiag / repadmin tools to again verify health is good.
--please don't forget to
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