Lost all levels and inventory without dying

When I launched Minecraft my character was at world spawn with no levels, no gear, empty inventory... but I hadn't died. I thought maybe I accidentally forgot to log off and had died,, so I played anyway.  I made new iron gear, got my diamonds out in my base and prepared to make an enchanting table. But I couldn't remember how many bookshelves I needed, so I left Minecraft and looked it up. When I relaunched Minecraft I was back at level 31 with my old inventory... and my diamonds have disappeared! Apparently, permanently, along with all the Iron I used for the new gear and all my iron which was still in my inventory. HELP!