I second Sam Freeman replies. A Mac will boot up and show you a message like "Your startup disk is almost full" but it won't refuse to boot. Your disk seems to have developed some issues. Is it acting straneg like clicking, ticking sound etc.? More importantly, do you have a backup of your Mac hard drive?
If backup exist then you can restore your Mac from Mac OS X Utilities and copy your data from the backup. Erasing the MBP drive will eradicate the file system problems. However, it will not work on a physically damaged hard drive.
FSCK - Restart your Mac and press-hold Command+S button while Mac is booting. You’ll enter single-user mode, which will provide you with a text-mode terminal. Type the following command into the terminal and press Enter to start a file system check: /sbin/fsck -fy
The command will run through several phases of checks. When it’s done, you’ll see a message saying “** The volume [name] appears to be OK” if everything is fine.
If it found problems, you’ll see a “***** FILE SYSTEM WAS MODIFIED *****” message. This indicates the fsck command found and fixed problems. The fsck command may find additional errors after repairing the first batch of errors, so Apple recommends you run the fsck command again if it found and fixed problems. Run the above fsck command over and over until you see a “** The volume [name] appears to be OK” message.
When the fsck command says your disk is okay, type the following command at the terminal and press Enter:
Your Mac will reboot, returning you to the usual login screen.