Hi Ali,
if you unlink your free application from the sandbox the application can still run locally. Note that the data is not necessarily shared. If you require data from the sandbox, those would also be gone after you delete that.
If you decide to bring the sandbox back*, you can just publish again.
read more about that here: https://docs.mendix.com/developerportal/deploy/licensing-apps/#unlink
* previous data will still be gone