Have you ever encountered sudden failure to RDP to your Windows desktop using Remmina? There are two known reasons to me:
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!