Don't worry! All your files are still stored on the hard drive.They're just invisible or inaccessible. You can recover them with hard drive data recovery software.
I suggest Tenorshare Data Recovery. I have used this one before on formatted hard drive, it works extremely well. Get a free trial from http://www.any-data-recovery.com/hard-drive-data-recovery.htm
And in future use Time Machine to back up all your files.
Hope I helped.