Can't boot MacBook Pro 13" (mid 2010)
It looks like you Mac doesn't detect an OS on the drive (or the OS is corrupted...) at least, that's what the crossed circle means. It's trying to boot, but it's missing some files in the System folder.

There are some solutions to this issue:

If your MacBook Pro is still able to "boot" (meaning, it reacts when pressing the power button):


Select your start-up disk:
Hold the alt button immediately after pressing the power button until some drives appear on the screen. Now select you Macintosh HD to start up from.
Try to force start from your system folder:
Press the X immediately after pressing the power button. It will force the Mac to start up from your Mac OS X. If this works, go to System Preferences and make sure your folder is the System Folder.
Boot from a bootable drive (like a Mountain Lion Startup USB disk) and re-install the OS. I sure hope you have a back-up of your files...


If your MacBook Pro isn't able to "boot" (same meaning as above):


Unplug the drive and connect it to another computer. Try to access the drive as an external hard drive, take a backup of your files and format the drive.
After that, plug it back in your MacBook Pro and use the last bullet of above to re-install your OS. 


Since I've been there and struggling for many hours to get things working, I think of myself as a semi-expert :)
So, if you've any questions at all, feel free to ask.