Before you go reinstalling all of OSX, try resetting your P-RAM. As soon as you press the power button, hold down Command+Option+P+R, until you hear the startup chime 3 times. Then release the keys and see if it boots up.
If this doesn’t work and you have to resort to reinstalling OSX, you should be able to select the option during install so that it will keep your accounts and user files intact. If it doesn’t offer this, you can do an “Archive & Install”, then drag over the files you wish to keep from the old system folder it creates.
If you want more info on this check out the mac support website on OSX installation options.