Of course, one should not panic and try to check what's wrong. In those cases, I usually run Testimo to quickly check relevant tests. While Testimo has more than 70 tests that can help you secure your environment it also is great for debugging or doing quick checks. It can save you a lot of time than trying to go thru your environment manually.
invoke-testimo -Sources DCDFS, DCServices, ForestReplication,DCDiagnostics, DomainLDAP
Within a few minutes, I got a full report that everything is mostly ok. Sure there are some small problems, but generally, replication works, services are up and running, dcdiag doesn't report anything major, and SYSVOL/DFS is mostly ok. In addition, I check some event logs, but nothing really stands out.