Monitoring/systemd unit state
If this alerts you should ssh to the server in question and run systemctl list-units --state=failed to check which unit is the one that has issues.
Try manually starting it with systemctl start <unit name>.
You can use systemctl status <unit name>, journalctl -u <unit name> and journalctl -xn to see more details and logs to figure out why it failed.
Sometimes the failure has been fixed already and you just need to clear the list of failed units with systemctl reset-failed.