Use this image as your inspiration!
What does it make you feel? Scared? Excited? Curious?
Do you have a key? Maybe it needs something more powerful than just a key.
Did you find the door somewhere forbidden or is it a door you see every day?
Are you on the right side of the door or have you gone to a place you cannot return from?
Are you alone, surrounded by others or searching for someone/something?
Are there consequences for your actions?
Be as creative as you can and let your brain wander through the possibilities! Use figurative language to bring your writing to life!