My RPi had power cut and after that didn't boot and this message comes on.
I put a memory card in to PC and made changes in cmdline.txt file
from /dev/mmcblk0p2 to /dev/mmcblk0p6 because I have seen this recommended for a similar problem with NOOBs. The error message then became unknown- block(179,6), and still no boot.
I also tried to hold Shift down but nothing changed.
There is no point in changing to p2 to p6 on a normal Raspbian install. Those are partition numbers starting from one and there are only 2, so p6 will never work. NOOBS uses additional partitions (and it is not impossible for the numbering scheme to have intentional gaps, but Raspbian does not -- see comments at bottom).
The original error is probably because the partition table was corrupted. If you have another linux machine or something you can run fdisk on stick it in that and look at the output of fdisk -l to see what happened. Unfortunately, unless you have a backup copy or the original image you installed from, figuring out how to set up/correct the partition table will be awkward because the boundaries have occasionally changed.
If you don't have something with fdisk, there are probably other tools that could be tried on any operating system, but again, you will have to use them to investigate the state of the partition table first.
Even if you do restore the table, there is no guarantee the data on the second partition will be okay and you should make sure to run e2fsck on it before you do anything else. However, the fact that the system still boots is a good sign -- the corruption might be very limited.
Had this same problem yesterday and used this tutorial and it worked fine.
You basically need to repair your filesystem which can be done so through recovery mode on a pi or through any other Linux or MAC based system.
