An Azure relational database service.
Hello Daichi
Check the overview page for the database on the Azure portal to see whether you have any remaining free credits for the month. If you choose the Auto-pause the database until next month option, once the free limits are met, the database in inaccessible until the start of the next calendar month. To continue usage with charges, go to the Compute + Storage tab and change the option to enable billing for usage of the database beyond the free amount.
https://learn.microsoft.com/en-us/azure/azure-sql/database/free-offer-faq?view=azuresql#help--i-m-no-longer-able-to-sign-in-to-my-database
Also, you could try restoring to a new database. Once the database is restored, test to see that it works okay, and then delete the old one. You do this via Backups blade, click Restore next to the restore point you want to restore, fill out options, Create, etc.
Could you please share the error message while trying to connect to the sql database.