The cause why you get this error when starting mplayer is that it chooses the “vdpau” output video driver as default. To change this one you have to change/add some lines in your config. file of mplayer. This may lie in /etc/mplayer or ~/.mplayer. Add this line to your config. file or alter the string behind the equal sign to your video driver:
vo=x11
You may also use your output video driver here, like xv for example. The nouveau video driver didn’t work for my part, but I think that X11 is in some way the “referrer” for it. To find out your driver use the command lsmod.
It’s possible that it got something to do with the GNOME Power Manager or it is much more simple, like in my case: there is not enough space left on your /home partition. Just delete a few files and you should be able to login again. If this doesn’t work out, invest your time in the gnome-power-manger package and it’s configuration.