a girl at work informed me she had a dream and she found me lying dead behind a door. She really shocked and scared me and this has affected me negatively. Can someone explain what this could mean and hopefully let me get on with living please?

Death in dreams usually represents change, so the dream could mean that she thinks that something about you is changing, or that she wishes that something about you would change.

"You" in the dream also might not be you, but an aspect of herself that reminds her of you, so "you" dying in the dream could represent something about herself that she wants to change or get rid of.

The door in a dream could represent a new path in life - moving on to new things.

Marcia's analysis is so well expressed. In dreams, death is not "the end", but transformation to something else. And it's important to remember that this was your friends dream, so it's about her and not you.