Lost progress and items.

I had progressed as much as defeating Draven and his consort, and Lachlan in "ACT 1 CRUELTY", once I logged out and logged back in, I had lost about 2-3 hours of progress and a bunch of items. I don't understand how this could happen, at first maybe it's got to do with my Ping but how did I manage to keep my Level? I was level 47-49 when I started these Acts but now I'm still level 51 without the progress and Items I picked up??? Somebody please explain and help,