If you are the subscription owner, you can simply go to the subscription and you can restore or replace the Service Administrator.
If you don't have owner RBAC, and you're global admin you can assign yourself the owner role and subsequent role you need. However, if there's no global admin you will need to contact product support.
https://azure.microsoft.com/en-us/support/create-ticket
If the above information was helpful please remember to 'Accept Answer' and 'Upvote'
Regards,
Abiola