What if my SQL Server evaluation period has expired?

Release candidates and community technology previews (CTPs) are considered evals and they expire 180 days after installation. You may not be able to upgrade the instance as upgrades are not supported (see Upgrading SQL Server 2019 from CTP to RTM version: Is it even possible?) Backup up your user database files (you'll have to copy the actual .mdf, .ldf, etc. files since you can't start SQL Server to do database backups), and then uninstall this instance, install the RTM version, and then attach the databases and create the necessary logins. Alternatively, of course, you could just install another instance with the RTM version, but then you'd have to connect to a non-default instance, etc.