MacBook Pro Won't Come Back On - HELP

Hi,

I'm hoping that somebody out there can help me!

When I turned on my MacBook Pro all that came up was a gray screen with a flashing folder and a question mark. I followed the instructions on the apple website on how to fix it (holding down the option/alt key when turning on etc.) and now it says " OS X Utilities" and gives the following options:

Restore from Time Machine Backup

Reinstall OS X

Get Help Online

Disk Utility

I have tried restoring from time machine backup but nothing happens - it says that it is searching for backups but nothing ever comes up.

I tried reinstalling a new copy of OS X Mountain Lion (even though I had already updated to Yosemite) but when I do that it asks which disk to install it on and then there is no disk to select.

I've tried to follow the information online but I'm worried that I am going to lose all of my documents and files that I have on my computer.

I'm hoping that somebody can please please help me - stress levels are running very high and I'm completely in the dark about what to do. Is my computer completely ruined?