Hi , hope this situation has been resolved by now.
for the next time , or otherwise if you still have the issue. This is a case in which AWS Support should be able to help you, because no general answer may apply and something contextual to your account might be causing the issue.
Your best next steps would be to open a Support case, in case you have not yet a Support subscription, consider to upgrade to Developer Support or Business support depending on the urgency of your case.
Without looking at the internal logs or having more detailed information on the situation it is not possible to provide guidance, sorry.
Alternatively, Please make sure you follow the best practices recommended by the Amazon open search to protect your cluster from getting in stuck state in future.
Best Practices: https://docs.aws.amazon.com/opensearch-service/latest/developerguide/bp.html
Hope this helps
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