The Windows Server Evaluation license is only valid for 180 days. Once the 180 days are up, things break. To extend this period by another 180 days, use the following command in PowerShell:
slmgr -rearmReboot the server to apply the rearm and re-validate your license for another 180 days.
The evaluation license can only be extended up to 3 years. After that point, I recommend standing up new installations of any existing servers and migrating to those and starting the whole evaluation process over again.