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.