Ubuntu file system becomes read only

My Ubuntu on Chromebook has this problem, and it fixed by $fsck -Af

Source Link is:
http://askubuntu.com/questions/197459/read-only-file-system

The filesystem will usually go into read-only while the system is running if there is a filesystem consistency issue. This is specified in fstab as errors=remount-ro and will occur when a FS access fails or an emergency read-only remount is requested via Alt+SysRq+U. You can run:

sudo fsck -Af

to force a check of all filesystems. As the other very good answer states, looking at dmesg is also very helpful.