Bonsai has a variety of cost controls that allow us to provide a quality free tier of service. Among these is the periodic deletion of unused free clusters.
Free clusters that have not been used for several weeks are added to a schedule for deletion. Warnings are sent out to users several days in advance of deletion. If the cluster receives traffic before its scheduled termination date, then the cluster is automatically removed from the schedule.
If you are an infrequent user of your Bonsai cluster, but wish to avoid it from being terminated, simply upgrade to any paid plan.