Source: How to perform a fsck (File System Check) on a system disk
Sometimes errors occur on the filesystem of your server's system disk that prevent it from working properly.
In this case, the only solution is to perform a filesystem check to (hopefully) fix the errors.
The fsck on a disk must be done while it is unmounted or in read-only mode. If not, the risk of losing data is very high! Always check the state of the disk before beginning the check and carefully read any warnings produced by the program.
There are two ways of proceeding with the check of the system disk depending on whether you have shell access or not.