Half or more of the Alertmanager instances within the same cluster are down.
You have an unstable cluster, if everything goes wrong you will lose the whole cluster.
Verify why pods are not running.
You can get a big picture with
$ kubectl get events --field-selector involvedObject.kind=Pod | grep alertmanager
There are no cheap options to mitigate this risk. Verifying any new changes in preprod before production environment should improve stability.