A volume Failed to Mount

[Barbiedoll]

The volume "Macintosh HD" could not be mounted.

What does this mean? I'm a newer Mac owner. My laptop seems to be working fine. A little slow sometimes but it starts and runs. but each time I start or restart, I get this message. What does this mean and what should I do to fix it?
Thanks

 

[iLog.Genius]

Try booting off your install disc that came with your computer and run Disk Utility. I'm willing to bet that you'll notice something that's not right (for example, on the left column where it lists your drives, Macintosh HD might be grayed out.)