You can install MS SQL as an evaluation version for 180 days, after which the SQL service will refuse to start, with an application log event saying the evaluation period has expired.
If the service is still running, but you're outside the 180 days, the edition upgrade freezes, because during the upgrade process it checks the SQL service and will stop it because its past 180 days, at which point the edition upgrade cannot continue because the service isn't running.
Microsoft provides a CLI command which allows you to activate it, but it too will either freeze or fails to actually activate SQL.
To fix this, if it's a domain attached SQL server, log in as a domain admin, then disconnect the network, then manually set the date back so it's back within the 180 day evaluation window. Then start the SQL service and do the activation through the GUI and it should work. You can check by going to properties for the SQL instance the SSMS. Once it's properly activated, you can set the clock back and reconnect the network.
