I had this model and both the springs were broken, this is a really poorly designed dish washer and the springs are two flimsy too take the weight of the door, and this is definately the cause of your problem. It is highly unlikely the spring slipped off
If it goes down really fast both springs are broken.
Fairly straight forward to fix once the casing comes off, and use some needle nose pliers. Make sure you spend the extra and use the genuine springs as I tried to replace with simlar springs and very difficult to replace the right size and tension.
I got the genuine springs from ebay (type in Fisher & Paykel 900T Series Door Spring # FP521565P) in the search part of ebay.com.au
good luck
Johnny
August 2009