Greetings all, total n00b here. I've recently put together a file server for my family and I've chosen to run Ubuntu on it. I set most everything up through SSH on a different computer (through much trial and error), so I could run the file server headless. Everything's been peachy for quite a while now, and the last time I SSH'd in was about a month ago. But yesterday, I tried to SSH in to make a few changes, and my SSH client went nuts telling me that the server's key had changed and that this usually signifies a man-in-the-middle attack.
Now, the server's on a local network, behind a router, with no ports forwarded to it, but still... this is kind of distressing. I haven't changed any kind of SSH settings, and in fact I'm pretty sure the file server hasn't even been powered off since last I SSH'd (although I could be wrong about that). Should I be worried? What should I do?