ive had this happen when Updates havent installed properly,
just boot off your system disk and run disc utility, you may have to reinstall.
if you have another computer handy, and if its a mac you can network boot...
plug in to LAN and hold N when booting up. you can then access it through your finder from another mac with password and username. i'd back up allmy files and re-install mac osx. sometimes the easiest way.
im sure you can do similar with another PC but i dont have any knowledge in that