This is not a question, but an answer.
Recently I've bumped into a problem when my microSD suddenly turned up write-protected.
Raspbian was unable to boot.
First thing I've done is googling
"raspberry sdcard becomes read-only", "raspbian root partition is locked / write-protected" etc.
9/10 of all answers suggest a hardware problem or moving the physical slider. Not in my case, the card was ok.
I also ran fsck, dosfsck, e2fsck several times - no effect.
So I couldnt easily find any relevant answer.....
If you happen to be in the same situation, try dropping to the Raspbian emergency shell and simply do:
cat /etc/fstab
In my case, the boot and root entries were commented out (by some kernel watchdog, NOT me):
proc /proc proc defaults 0 0
#/dev/mmcblk0p1 /boot vfat defaults 0 2
#/dev/mmcblk0p2 / ext4 defaults,noatime 0 1
Stick your card into another Linux machine, mount the root, uncomment them and save.
Did it work?
Hope this will help!