Code Property – Software Engineering Stack Exchange

I have written an extensive code using C # and SQL for a volunteer organization. I have done it completely on my own initiative. I am the soul developer of this code. I established requirements, procedures and all the relevant information. It has been in use by a non-profit organization for approx. 7 years. I have not copied it. The nonprofit organization has not paid for the use of it.

Now there is a question of ownership. They have copied it to a new server, they are using it in the new server, they have made some minor modifications and they have denied my access to the new server.

Who owns this code and who has rights over the source code?

Thank you