I have a macbook that I use on occasion which dual boots Ubuntu and OSX (It has rEFIt installed). I turned it on for the first time in a while and it needed a bunch of updates. So I let it run, and restarted it when it asked. When it was booting up, it got stuck at a light blue screen. There was nothing on the screen to indicate that it was doing anything - I figured it just got stuck or something, so I turned it off and back on. (I suspect now it was actually working, but I had no indication that it hadn't just frozen)
Now I can't access either OSX or my Ubuntu partition. When I choose ubuntu on the rEFIt menu, it shows "No bootable device -- insert book disk and press key". If I try to start up OSX is looks like it starts loading, but instead of an apple logo there's a crossed out circle icon.
Edit: A bit spooky, but the problem seems to have gone away. No idea why. I tried booting up again under OSX so I could see if there was any diagnostic information I could google, but this time the desktop came up without a problem. I thought that was a bit odd, and decided to try the Ubuntu partition as well, which now also works just fine. I'm now even more suspicous of hardware problems... Luckily there's nothing terribly important on this computer, but I'll use it as long as it works I guess.