Q: My mac doesnt start!!!

hold the Option key. Somehow either the boot drive won't mount and has a bad directory from crashes and errors or the path to the drive was changed and it needs to be reset in Startup Disk control panel.

Use the Lion Recovery Mode to repair the disk w/ Command + r when you boot in Disk utility and you can set the default boot drive - Startup Disk - as well.

you should have backups and be able and know how to restore and be ready at any time before it strikes.

General purpose Mac troubleshooting guide:

Mac OS X: Starting up in Safe Mode - http://docs.info.apple.com/article.html?artnum=107393

What is Safe Boot, Safe Mode? (Mac OS X) - http://support.apple.com/kb/HT1564

Isolating issues in Mac OS X

Creating a temporary user to isolate user-specific problems:

Isolating an issue by using another user account

Identifying resource hogs and other tips:

Using Activity Monitor to read System Memory and determine how much RAM is being used

Starting the computer in "safe mode":

Mac OS X: What is Safe Boot, Safe Mode?

To identify potential hardware problems:

Apple Hardware Test

General Mac maintenance:

Tips to keep your Mac in top form

Direct you to the proper forum for MacBook :

MacBook Series Forums

https://discussions.apple.com/community/notebooks?view=discussions

http://www.apple.com/support/macbookpro

Mac OS X Forum

https://discussions.apple.com/community/mac_os?view=discussions

Recovery Mode

http://support.apple.com/kb/HT4718

TimeMachine 101

https://support.apple.com/kb/HT1427

http://www.apple.com/support/timemachine

Mac OS X & Mountain Lion Community

https://discussions.apple.com/community/mac_os

https://discussions.apple.com/community/mac_os/os_x_mountain_lion?view=discussio ns

This forum deals with a desktop/tower 65lb Mac Pro

http://www.apple.com/support/macpro