How can I fix back door on 2000 Cheverolet Astro Van?

The door/window that you lift up to access the space behind the last row of seats in my van has somehow lost its ability to stay up once you open it. It's like a spring or hinge isn't catching. I have to manually hold the door up. If I'm not careful it slams down. Any idea what might be causing this and how to fix? Single mom of four, not terribly familiar with mechanics, so laymen terms please! :)