Something similar happened in my student house a few yrs back. We all left to go home for our 2wk easter break and the last one out left the door unlocked ie turn the handle and walk right in! :s When I got back I was a bit miffed at the prospect of a burglary, but luckily everything was as we left it.
I knew who it was so I had a quiet word, along the lines of "when you left, you forgot to lock the door. We were lucky this time, but try not to do it again". And that was the end of it.

IMO the silent treatment is not a mature way for your roommate to deal with this situation. We all make mistakes. I'm sure her reaction won't be as bad as you fear. I frequently find that the fear is WAY worse than the reality ever is

*Willow*