A likely explanation is that there is a hardware problem (partial disk failure), and that the kernel remounted the root filesystem as read-only as soon as it detected the problem, in order to minimize the problem. A more reliable¹ way to check current mount options is cat /proc/mounts (grep ' / ' /proc/mounts for the root filesystem, ignore a rootfs / … line which is an artefact of the boot process). You will presumably find that rw,errors=remount-ro has changed to ro (other options may be displayed in addition).

Check the kernel logs (/var/log/kern.log) for the message Remounting filesystem read-only. It is typically preceded by disk access errors.

As an aside, under Ubuntu, the usual place for mount points (used by the desktop interface) is under /media (e.g. /media/cdrom0), though you can use /mnt or /mnt/cdrom if you like.

¹ _{mount reports from /etc/mtab. If the root filesystem is read-only, /etc/mtab can't be kept up-to-date.}

A likely explanation is that there is a hardware problem (partial disk failure), and that the kernel remounted the root filesystem as read-only as soon as it detected the problem, in order to minimize the problem. A more reliable¹ way to check current mount options is cat /proc/mounts (grep ' / ' /proc/mounts for the root filesystem, ignore a rootfs / … line which is an artefact of the boot process). You will presumably find that rw,errors=remount-ro has changed to ro (other options may be displayed in addition).

The kernel logs probably contain the message Remounting filesystem read-only, preceded by disk access errors. The logs normally live in /var/log/kern.log, however if this is on a now read-only filesystem the message will not show up there, though the preceding errors should. You can also see the latest few kernel errors with the dmesg command.

As an aside, under Ubuntu, the usual place for mount points (used by the desktop interface) is under /media (e.g. /media/cdrom0), though you can use /mnt or /mnt/cdrom if you like.

¹ _{mount reports from /etc/mtab. If the root filesystem is read-only, /etc/mtab can't be kept up-to-date.}