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
Creating a temporary user to isolate user-specific problems:
Identifying resource hogs and other tips:
Starting the computer in "safe mode":
To identify potential hardware problems:
General Mac maintenance:
Direct you to the proper forum for MacBook :
MacBook Series Forums
Mac OS X Forum
Mac OS X & Mountain Lion Community
This forum deals with a desktop/tower 65lb Mac Pro