Hi Eniyan,
Probably, your Mendix Studio Pro closed without being able to stop the App from running.
You can do two things:
1. Either restart your PC.
2. Or, you can kill the port (I assume you were using port 8080) by following the instructions here:
https://stackoverflow.com/questions/39632667/how-do-i-remove-the-process-currently-using-a-port-on-localhost-in-windows