Q: i Mac shutting down on startup

Try a Safe Boot Try safe mode if your Mac doesn't finish starting up - Apple Support

About the screens you see when your Mac starts up - Apple Support

Maybe I don't understand when you say "the desk is locked". The page I linked to has a picture of a screen with a padlock on it, is that what you're seeing?

To reinstall OS X restart while holding the Command-R keys to access the recovery partition. Choose reinstall OS X. You  should have a complete backup of your system before proceeding just in case something goes wrong.

There is always a chance of losing your files, in fact if you don't already have a backup you're taking that chance every day. Since your Mac won't start the only way I know of to back up your files is to start it up in Target Disk Mode How to use and troubleshoot FireWire target disk mode - Apple Support and connect it to another Mac and use Disk Utility or a third party app like Carbon Copy Cloner to make a bootable clone of your internal hard drive. If you  do not have another Mac available or your Mac won't boot into TDM then I don't know how you would back it up.

I will add that while I've always had a backup before upgrading or reinstalling OS X nothing has ever gone wrong for me, I've been fortunate enough to have never lost files.