There is no direct option to restart an Azure Managed Grafana instance.
I will recommend you your instance's metrics and logs in the Azure portal for any anomalies. A possible option for you will be to scale down and scale up your grafana instance as a troubleshooting step.
In a worst case scenario you could also export your instance settings and recreate.
You can use the CLI to do these: https://learn.microsoft.com/en-us/cli/azure/grafana?view=azure-cli-latest
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Regards,
Abiola