Since by design, you cannot recover from a CA root certificate expiring, sometimes you need to limp along, and continue to issue certs even though you cannot necessarily revoke them, because the CRL published in Active Directory is now incorrect, or offline.
While we can argue all day about the benefits/detractors of this, here it is:
To bring the CA Root back online after the Root certificate expires, issue these commands in an elevated powershell:

certutil –setreg ca\CRLFlags +CRLF_REVCHECK_IGNORE_OFFLINE
net stop certsvc && net start certsvc

Now, go back to the drawing board, and PLAN your PKI implementation, and DON’T LET YOUR CA ROOT CERTIFICATE EXPIRE!!!

Incidentally, once you’ve fixed your certificate snafu, to stop ignoring offline CRLs, do this in an elevated command prompt:

certutil –setreg ca\CRLFlags -CRLF_REVCHECK_IGNORE_OFFLINE
net stop certsvc && net start certsvc