I know! It's preety upsetting when this happens!
Give her 15 days to pick it up. Get it out of your sight if possible.
Contact her with whatever means you have: email, instant messenger, phone, friends, family or letter.
Repeat the same message a few times to make sure she gets it:
"You have 15 days to come pick up your stuff. After that, I'll simply get rid of it. I did not hear from you and you are obviously gone forever."
It's her responsibility to take care of her things, not yours. So, you don't have to feel responsible for it. You are not a storage warehouse.
Be fare and give her a chance to get it back if she wants to. However, if she does not care about her belongings, why should you?
Will she freak out if her stuff is gone when she wants it back 6 months from now? Maybe... Who cares.
She did not care much when she took off without warning. Did she worry about you or what would happen to you next? Not at all, right?
This is a radical solution but it works and it is your right. It's your house.
You don't have to sit around with her stuff! It does not belong to you.
To your power!