As someone who already has been bitten by this.
If you are running crouton with the chrome beta channel, and it is still working, I recommend switching back to stable channel right now before the next update. It seems that changes to beta have broken the ability of xorg to run for crouton. Here is the bug report:https://github.com/dnschneid/crouton/issues/524
If you already have this problem, the top link has some workarounds (that sadly disable hardware acceleration, but do get things working). Also, if you feel compelled to star the bug to get it higher in the pain list, please refrain from sending nasty comments through in the replies. As a developer myself, there is little that is less motivating than a lot of rude, angry messages. I believe a fix is coming, but now I wish I had acted on my impulse earlier in the week and switched to stable before this happened.