multiple things you have to verify, 1. check if the rdp is running on the selected vm. 2. if the vm been moved or you accidentally moved it to a different datastore or even deleted. 3. try to elaborate on your setup -- for example if you have a dhcp server and the selected vm has not been online for sometime then the dhcp server will release the ip. try to access the selected vm by the hostname. 4. check if the vm VNIC is running and no issues present. give us more info about your setup so we can help.