After demoting a domain controller to a member server, you may be greeted with At least one service or driver failed during system startup. Use Event Viewer to examine the event log for details when you restart.
If you configured the service to start with an account other than the local system account, it is nolonger available.
The demotion of a DC creates a new SAM, with a new Administrator account and other new builtin users and groups.
The only solution is to change the service account, using Administrative Tools / Services.
For Exhange Server, see Q163686 XADM: What to Do If the Service Account Is Deleted.
NOTE: When you promote a member server, all user accounts and groups are incorporated into the Active Directory, so this doesn't happen.