When you install ssh on a server, a host key is created. The message you get is telling you that this key has changed. Usually this happens when someone does a reinstall of the server. It could also be that there is something happening, like DNS spoofing.
The computer (operating system) you reach when you do a "ssh Computer-NAME" is not the same as last time you connected with ssh. If you know the reason (like if there has been a reinstall), delete line 3 in the file "/home/USERNAME/.ssh/known_hosts", and try again. If not, I would contact the administrator of that server and ask what's going on.