How I Lost My Files of 5+ Years

Okay, lately, my computer has been acting really sluggish. Yesterday, I was tired of it, so I saved all of my files onto my sister's hard drive, so I could run a system restore. My computer came with a disk to do a restore, so I did that, but I made a fatal error. I left in external hard drive (H:) connected to my computer. I noticed this while I was running the restore, but it was still on (C:), so I didn't think it would effect the hard drive, but I looked today, and it did! Needless to say, I am VERY frustrated.