- your RDP plugin for Reminna is missing. In Fedora, type sudo yum install remmina-plugins-rdp, or Ubuntu sudo apt-get install remmina-plugin-rdp. You may require to reboot after install in order to load the RDP plugin. Sometimes Import simply won't work.
- your remote desktop's fingerprint has changed. It is impossible to reset the fingerprint within Remmina's interface. What you can do is fire up terminal, go to ~/.freerdp/ and delete known_hosts.
Now try again and voila! Your remote desktop is back!