As I was studying redhat, I had to play round with /etc/fstab file. And the lab I was working on said
'comment out the line that defines the root partition in /etc/fstab.'
So I commented out that line and rebooted.
After I go into the maintanance shell by typing init=/bin/sh in kernel command in GRUB, I type
mount -o remount,rw /
to remount root in read,write to uncomment the line in /etc/fstab.
But it says it can't find / in /etc/fstab. (obviously..I just commented it out)
I can't editthe /etc/fstab directly because it's read only file. So I don't know what to do at this point. According to the lab, remount should have no problem.

Did I screw it up and should I re-install OS?
Sorry about my bad explanation. can't explain very well

Not sure you did exactly what you thought, but I can recommend backing up files BEFORE editing (esp key stuff like fstab)

Anyway, grab the install media, boot from that and select the Rescue option, which should eventually mount the HDD under /mnt/sysimage & you go from there.
(NB: check the mount first; it may be ro or rw; I can't remember off-hand)

Thank you for the reply, Chris.
I did copy my original fstab as /etc/fstab.prelab
But I can't replace my original with backup copy.
I tried to overwrite the original by typing ' cp /etc/fstab.prelab /etc/fstab '
Then it said it can't process because /etc/fstab is read-only file.

Did you do what I said ie reboot with install media etc?

The lab is wrong. That (short) form of the mount command will only work if the mountpoint is defined in fstab - which, like you said, it isn't any more.
Issue the command with the device specified as well - just cat fstab to get the correct specification. Something like (might be UUID or label instead - see your fstab).

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