My Kitchen aid dishwasher door is suddenly heavy and drops open. Seems like the hinge is broken. How can I fix it?

First thing is to identify the faulty part, sometimes this can be done from the front bottom panel on newer appliances you may have to take the side panels off,( remember to turn power off)
Contact manufacturer quoting model and serial number and ask them to send you the door balance parts( a good idea to change both as the surviving one has been over stressed).Most manufacturers will send out parts by post

Sometimes it is just a realignment issue,the spring can be placed in many slots and could have just slipped off,check by removing the kickplate(with power off) and looking beside the unit,if you can't see the spring base remove the screws on the top(under the countertop) which hold the unit in place,pull the entire unit out and look at the springs(this could save you a service call from a professional)

It could be a problem with a spring. We had a similar problem and had someone who knows <a appliance repair in toronto, come look at it to find out it was just a broken spring. It wasn't hard to replace and didn't take long at all. Maybe that could be it? http://www.commonwealthappliances.ca/