SSRS is part of SQL Server, it doesn't have separate licensing and hasn't for many years. You don't buy SSRS licenses.
How did you figure it out "had expired"? My gut instinct is that it is your SQL Server licensing. However I'm not aware of a subscription-based SQL Server licensing model (other than a trial license) either so it wouldn't just expire.
Did you happen to upgrade your SQL Server from an older version to a newer version? If so then you'd need updated CALs to talk to SQL. If you were using a trial license of SQL then you'd need to update that instead. The instructions for doing so are given here. This would license your SQL Server instance.