My Macbook does not bootup!

Symptom:
Your macbook gets to the wheel spinning page and it shuts down.

Solution: (Stop on any step that you get results!)
1) Turn off your mac by holding the power button for 5 sec. Restart
2) Try resetting PRAM by holding Option-Command-P-R during the boot, Remove your hands after you hear the starting sound for the second time. It will Restart
3.1) Hold Command-S during boot
3.2) Type df
3.3) look for the name of your drive. Most likely it is /dev/disk0s2
3.4) Type fsck_hfs -r /dev/disk0s2
3.5) If it exited with errors after 3 trials run it again until you get successful results.
3.6) Restart
3.7) Backup Data!

PS: More information about solution 3 is here.
PPS: If you could not get out of loop 3.5 try rebooting your system in between. If you get into Mac OS X. Backup immediately!