oh-no
let's run through some potential issues
Broken disk or failure to mount
just run this code
fsck /dev/whatever-disk-name
wrong directory chowned or chmoded
just run the command that'll reverse the effect (just don't be stupid and chown /
)
rm'd bootloader
idiot. that's all i have for that one...
look through your backups and remeber the good old days... you could also copy your home directory to an external drive and safe that