The possible cause is corrupt filesystem or wrong entries in /etc/fstab.
When system comes into the state where it has file system mounted as read only.
issue command mount -w -o remount / as root and then check for possible wrong entries in /etc/fstab or fsck on boot to check filesystem.
In most of the cases the issue is solved by the above steps.
if the issue is not solved then HDD needs to replaced 
Naren
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