You can check the reason must be one of the following:
- First check you are using correct IP address and port number to login into RDP.
- Default port of windows RDP is 3389 so check this port is not blocked on your firewall or ISP. And if you are using any configured port then you have to check it to.
- Also this is silly but sometimes we are using incorrect username and password, so we must have checked it.
- You can also use telnet command to check your connection.
- Here you have to use your server IP address instead of IP-address and your server port number instead of port.